[Concepts] Concept Specialization Expressions

[saar.raz]

Summary

Part of the P0734r0 Concepts implementation effort. Added Concept Specialization Expressions and tests thereof. Also added constraint expression type verification. This diff depends on D40381.

[changyu]

lib/AST/ExprCXX.cpp

1680

You have four spaces of indenting here.

lib/Parse/ParseExpr.cpp

241

Indenting.

[saar.raz]

• Fixed indentation problems

[saar.raz]

Previous updated diff mistakenly included D40381, fixed that.

[saar.raz]

Fixed indentations.

[faisalv]

lib/Parse/ParseExpr.cpp

239–240

Are you sure this only accepts logical-or-epxressions? Unlike Hubert's initial implementation here, this seems to parse any expression (i.e including unparenthesized commas and assignments)? What am I missing?

[saar.raz]

lib/Parse/ParseExpr.cpp

239–240

Mm.. Good point, I was under the false assumption that this was equivalent - I'll fix it back, thanks :)

[saar.raz]

Reverted to original constraint parsing code.

[saar.raz]

• Moved general concepts-related function into SemaConcept.cpp

[• Quuxplusone]

include/clang/Basic/DiagnosticSemaKinds.td

2470

Extra period . here

lib/Serialization/ASTReaderStmt.cpp

3474

Peanut gallery says: All the other cases look like

S = new (Context) FooExpr(Context, Empty);

or

S = new (Context) FooExpr::CreateEmpty(Context);

not

S = new (Context) FooExpr(Context);

Is the visual difference in this case significant?

test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

12

"test/Parser/cxx-concept-declaration.cpp" has some syntax tests for "value-concepts" of the form template<bool x> concept.... Would it make sense to add some semantic tests for template<bool x> concept... in or near this test file?
(This could plausibly be "out of scope." I merely mention it.)

test/Parser/cxx-concept-declaration.cpp

36 ↗

(On Diff #132743)

Peanut gallery says: IIUC, C12<T> is satisfied whenever T has a constexpr constructor and a constexpr implicit conversion to bool that returns true? Or (because this is an atomic constraint and atomic constraints don't do conversions, as we can see on line 31 above) is C12<T> *actually* never satisfiable (but there's no diagnostic expected here because T{} is dependent)?

[saar.raz]

test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

12

Good idea, I'll add some of those.

test/Parser/cxx-concept-declaration.cpp

36 ↗

(On Diff #132743)

C12 will only accept bool, and there's no diagnostic because it's dependent is correct.

[saar.raz]

• Addressed comments: Added semantic tests for value and template concepts, removed extra period and modified CSE ctor to fit in nicer.

[saar.raz]

• Fixed incorrect checking of atomic constraint types.

[faisalv]

include/clang/AST/ExprCXX.h

4725

Typo (Hubert): concepts -> concept

4728

Hubert: The concept named.

4729

NamedConcept

include/clang/Sema/Sema.h

5899

Hubert: clang-format this

5902

Hubert: Typo: emitted

lib/AST/ExprCXX.cpp

1640

Hubert: clang-format

1677

Hubert: This needs a TODO: the idea is not to drop SFINAE errors, but to avoid instantiation that may trigger errors not in the immediate context of instantiation. The substitution needs to happen piecewise.

1684

Hubert: The name of the function gives no indication that it modifies the expression node. The interface of the function extends past what is expected.

[saar.raz]

lib/AST/ExprCXX.cpp

1677

Could you elaborate/give an example where this handling is inappropriate?

[saar.raz]

Addressed most comments.

[saar.raz]

Applied missing clang-format.

[hubert.reinterpretcast]

lib/AST/ExprCXX.cpp

1677

Determining satisfaction requires normalization (lazy normalization should be considered).
The determination of satisfaction then proceeds by handling the left-hand side of conjunction and disjunction constraints before possibly substituting into the right-hand side; i.e., there is short-circuiting behaviour.

Example:

template <typename T>
concept C = true;

template <typename T>
struct Q { static constexpr T value = nullptr; };

template <typename T>
requires C<T> || T::value
struct A { };

template <typename T>
requires C<T> || Q<T>::value
struct B { };

A<int> a; // okay
B<int> b; // okay

[saar.raz]

lib/AST/ExprCXX.cpp

1677

OK I see your point. You said this should be a TODO - do you think this should be delayed to a further patch (namely D41569, where we actually deal with diagnostics and stuff)?
About lazy normalization, I tend to think it is not a good idea - you'll probably check satisfaction for every defined constraint expression at some point, and do that many more times than you'd define a new one

[hubert.reinterpretcast]

lib/AST/ExprCXX.cpp

1677

The status quo of this patch with regards to this topic is not quite something that I would like to commit onto trunk.

As for normalization: The process is needed for distinct purposes with somewhat differing output requirements.
The order-dependent form (to check for satisfaction) can be integrated with the substitution and evaluation: that is, the output of normalization is realized in control flow, and not "stored".

The order-independent form (to check for subsumption) could be delayed until overload resolution with a constrained candidate reaches the point where determining subsumption is necessary.

[saar.raz]

• Switched to piecewise substitution of atomic constraints when evaluating constraint expressions
• Added tests for piecewise substitution behavior
• Removed some redundant parentheses

[saar.raz]

• Switch to ASTTemplateArgumentListInfo, add ConstantEvaluated context when parsing constraints

[rsmith]

include/clang/AST/ExprCXX.h

4729

You should also track the FoundDecl and the optional NestedNameSpecifierLoc (just like a DeclRefExpr would). Clang-based tools (particularly refactoring tools) need those. There's also an optional template keyword, but it can never matter in practice because the nested name specifier can never be dependent, so it's probably not the most high-priority thing to track.

4732–4733

It'd be nice if at least one of these two arrays could be tail-allocated.

4768–4772

Passing Sema in here is a major layering violation. Here's how this should work:

• The AST code represents the AST model; its functions should generally not be able to fail.
• Sema checks the semantic constraints and enforces the validity of operations, and updates the AST node as appropriate

In this case, Sema should be checking the template argument list as written against the concept's parameter list, forming a resolved list of template arguments, and then passing both the as-written information and the resolved arguments to this AST node for storage.

4783–4790

Do you really need mutators for the 'satisfied' flag and the concept name location?

include/clang/Basic/DiagnosticSemaKinds.td

2471

Find a way to describe this problem without pretty-printing a substituted expression.

2473

Remove the '%0' here; you're already underlining it with a SourceRange (right?).

Please read http://clang.llvm.org/docs/InternalsManual.html#the-format-string and in particular "Take advantage of location information. The user will be able to see the line and location of the caret, so you don’t need to tell them that the problem is with the 4th argument to the function: just point to it."

2475

There's not really any such thing as a "concept specialization". Perhaps "while checking satisfaction of '%0'" or similar?

lib/AST/ExprCXX.cpp

1659

You will need to skip over a top-level ExprWithCleanups (I can't think off-hand of any other nodes that could show up here, but there might be more).

1751–1752

Build a CodeSynthesisContext for this rather than adding an ad-hoc note to the last diagnostic that happens to come from substitution (which might not be the error you care about, and might even be suppressed by warning flags).

lib/AST/ExprConstant.cpp

11248–11250

You don't need to check for this. If the evaluator is called on a value-dependent expression, that's a bug in the caller.

lib/AST/StmtPrinter.cpp

2223

You should print out the nested name specifier here. (And ideally also the template keyword, if specified...). And you should print the name of the found declaration, not the name of the concept (although they can't differ currently).

lib/Parse/ParseTemplate.cpp

374–377 ↗

(On Diff #159234)

Remove comment and braces. We don't need to say that an invalid ExprResult means a diagnostic was produced, because that's always implied by the ExprResult being invalid.

lib/Sema/SemaConcept.cpp

23

* on the right, please. And replace BinaryOperator with auto since it's initialized by a dyn_cast.

25–26

&& on the previous line, and align the two operands. (Or get clang-format to do it for you.)

35

What justifies rejecting this prior to any use of the concept that would result in a satisfaction check?

(I think checking this is a good thing; what I'm wondering is whether we need to file a core issue to get the wording updated to allow us to reject such bogus concepts even if they're never used.)

38–42

Call IgnoreParenImpCasts before checking the type of an atomic constraint, rather than adding an extra recursive step here.

123

Please fix :)

[rsmith]

lib/Sema/SemaExprCXX.cpp

7690 ↗

(On Diff #159234)

Please fix.

lib/Sema/TreeTransform.h

3006–3019

We need to convert the template arguments when rebuilding. This should call CheckConceptTemplateId instead. (After fixing that, please inline CreateConceptSpecializationExpr into its only caller.)

Please also add a test that checks we do convert the template arguments here.

[saar.raz]

@rsmith - thanks for the responses!
Will also address the comments I haven't responded to soon.

include/clang/AST/ExprCXX.h

4729

Isn't the FoundDecl just the NamedConcept?

4732–4733

Now that you mention it, TemplateArgs is never even used or set, I'll remove it.
As for TemplateArgInfo - do you mean allocate the ASTTemplateArgumentListInfo itself as a trailing obj or the individual TemplateArgumentLocs? the former seems more reasonable to me as it's convenient to have an ASTTemplateArgumentListInfo object around

4783–4790

They're here only because of ASTReader/Writer. I guess I can use a friend decl instead.

lib/AST/StmtPrinter.cpp

2223

Are there possible upcoming changes that'll make them differ?

lib/Sema/SemaConcept.cpp

35

I guess this is already justified, if awkwardly (NDR) by [temp.res]p8.2

[saar.raz]

lib/Sema/SemaConcept.cpp

35

Correction - it says that IFNDR occurs when no substitution would result in a valid expression, so maybe this is well formed after all.
In this case it is a valid expression but not a valid constraint expression, maybe that's the missing word here?

[saar.raz]

Address Richard's CR comments. Among other things:

• CSEs overhauled and now store both source and converted template arguments (latter are tail-allocated), template KW location and NNS.
• CSEs no longer violate layering - satisfaction check now moved back to CheckConceptTemplateId.
• CodeSynthesisContexts created for both the act of checking the associated constraints of a template or the constraint expression of a concept (non-SFINAE), and for the act of substituting template arguments into a(n atomic) constraint expression (SFINAE).
• Added more tests for cases where substitution leads to a non-SFINAE failure and emits diagnostics
• Fixed and added test for incorrect conversion of instantiated CSE argument list.

[saar.raz]

Removed unused "note_in_concept_specialization" diagnostic ID.

[• Quuxplusone]

include/clang/Sema/Sema.h

7134

Missing a trailing comma here.

test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

121

Nit: You could use // expected-note@-1{{...}}, // expected-note@-2{{...}} to make lines like this more readable.

test/Parser/cxx-concept-declaration.cpp

48 ↗

(On Diff #160237)

Should you static-assert the expected results?

static_assert(!C12<bool>);
static_assert(C13<int>);
static_assert(C14<int>);
static_assert(C15<int>);
static_assert(C16<true>);
static_assert(C16<false>);
static_assert(C17<true>);
static_assert(!C17<false>);

[saar.raz]

test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

121

😮😳 Well you learn something every day

test/Parser/cxx-concept-declaration.cpp

48 ↗

(On Diff #160237)

That's not the point of these tests but it couldn't hurt, I guess

[saar.raz]

Address Arthur's comments, add missing CorrectDelayedTyposInExpr

[steveire]

include/clang/AST/DeclTemplate.h

3063 ↗

(On Diff #160437)

getLocEnd is deprecated and will be removed soon. See http://clang-developers.42468.n3.nabble.com/API-Removal-Notice-4-weeks-getStartLoc-getLocStart-getLocEnd-td4061566.html

[saar.raz]

include/clang/AST/DeclTemplate.h

3063 ↗

(On Diff #160437)

Good to know, but I'm not yet merged against trunk so getEndLoc isn't here yet (will soon post a merged version of this patch)

[saar.raz]

• Fix bad ArgsAsWritten assertion, add missing null initializer in ConceptSpecializationExpr.

[rsmith]

include/clang/AST/ExprCXX.h

4729

Not in general, no. The FoundDecl could be a UsingShadowDecl.

lib/AST/ExprCXX.cpp

1671

This is incorrect. Whether the expression is dependent is not the same thing as whether it's instantiation-depnedent. A concept specialization should be considered instantiation-dependent if it has any instantiation-dependent template arguments, and should be considered value-dependent if it has any dependent template arguments.

For example, Concept<sizeof(sizeof(T))> has an instantiation-dependent template argument, but no dependent template arguments, so should be instantiation-dependent (the validity of the construct can depend on the type T) but not value-dependent (the value of the construct is always Concept<sizeof(size_t)>, which doesn't depend on T).

lib/AST/StmtPrinter.cpp

2223

p0945r0 would allow them to differ.

lib/Sema/SemaConcept.cpp

106–113

Please collect the notes from EvalutaeAsBooleanCondition and emit them here, so the user knows /why/ the expression was non-constant.

lib/Sema/SemaTemplate.cpp

4257–4261

Add braces around this for, because its body contains multiple statements. (Generally, if an inner construct has braces we want outer constructs to have them too.)

lib/Sema/SemaTemplateInstantiate.cpp

697–699

Is this note ever useful? It will presumably always point into the same concept definition that the prior diagnostic also pointed at, and doesn't seem to add anything in the testcases.

Maybe we could keep the CodeSynthesisContext around as a marker that we've entered a SFINAE context, but not have any corresponding diagnostic. (The note produced for the enclosing ConstraintsCheck context covers that.) Or we could remove this and store a flag on the ConstraintsCheck to indicate whether we're in a SFINAEable portion of it.

[saar.raz]

lib/Sema/SemaTemplateInstantiate.cpp

697–699

If the concept definition is multiline/contains macros, this would point at the exact place where the problematic constraint occured, we should probably add tests for this case though.
Maybe we can omit the diagnostic when the concept and the constraint are on the same line or something?

[saar.raz]

lib/Sema/SemaTemplateInstantiate.cpp

697–699

Correction - I just now realized you were referring to the 'in instantiation of template class' note and not the 'while checking the satisfaction...' note.
In this case - the only case I can think of where the problematic instantiation and the constraint expression would be in very different places is indeed multiline constraint expressions or macros in a constraint expression (but you know better than I do - can you think of any other cases? or maybe there are other non-SFINAE errors that can result from substitution and would not produce a note?). Maybe these cases are remote enough to warrant removing this diagnostic or as I said earlier omit them in cases where they are unhelpful.

[saar.raz]

• Address CR comments - add FoundDecl tracking, instantiationdependent calculation, display non-constant expression diagnostic notes

[saar.raz]

Moved from getLocStart, getLocEnd to getBeginLoc, getEndLoc. Rebase onto master/trunk.

[saar.raz]

Add new CodeSynthesisContexts to switch where they were missing

[saar.raz]

Fix wrong patch uploaded

[saar.raz]

Add support for CSE mangling

[saar.raz]

Delay support for mangling to later patch (where CSEs are formed instead of UnresolvedLookupExprs with dependent args)

[saar.raz]

Rebase onto trunk.

[rsmith]

include/clang/Sema/Sema.h

5904

(Nit: please align the second and subsequent parameters here with the first one.)

7487–7488

It'd be useful to note that this context covers the check that atomic constraints have type bool and can be constant-evaluated. (It's not obvious how we'd hit diagnostics in this case, since most substitution failures result in an unsatisfied constraint.)

lib/AST/ExprCXX.cpp

1703–1704

It'd be nice to assert that the nested name specifier is neither instantiation-dependent nor contains an unexpanded parameter pack. (That's true today because the NNS of a concept specialization expression can only refer to a namespace.) If the NNS could be instantiation-dependent or contain an unexpanded pack, we should inherit those properties here.

1708–1709

This code would be a little clearer if you moved an if (IsDependent && ContainsUnexpandedParameterPack && IsInstantiationDependent) check to the end of the loop. (I doubt the change will ever make any performance difference.)

lib/AST/StmtProfile.cpp

1303–1305

It seems a little inconsistent to use the named concept rather than the found decl (that is, resolved rather than as-written), but to use the as-written template arguments rather than the resolved and converted arguments. (I don't think it matters either way, since the profiler is in principle allowed to use either the as-written or resolved form, but generally it should prefer to use the as-written form.)

lib/Sema/SemaConcept.cpp

29

Nit: indent the second line so that the two CheckConstraintExpression calls line up.

35

OK, I'll take this to the core reflector to make sure the wording is clear this is ill-formed, no diagnostic required.

38–42

I think we're missing this, and we won't reject:

template<typename T> concept bool X = true && (0 && 0);

because we treat (0 && 0) as an atomic constraint rather than treating the two 0 subexpressions as atomic constraints here.

(Maybe add ConstraintExpression = ConstraintExpression->IgnoreParenImpCasts(); or something like that to the start of the function?)

51–68

If an operator&& or operator|| function is declared, this could be a CXXOperatorCallExpr instead. You will need to recurse into those constructs too.

69–71

Consider using IgnoreParenImpCasts here rather than manually handling these cases. (We should be consistent in the mechanism by which we traverse constraint expressions.)

This will do different things particularly for certain language extensions, such as _Generic, __builtin_choose_expr, and __extension (which are treated as being parens-like).

72–74

This case is not handled by CheckConstraintExpression; we should be consistent in whether we step over these or not.

Perhaps it would make sense to factor out a mechanism to take an expression and classify it as atomic constraint (with the corresponding expression), or conjunction (with a pair of subexpressions), or disjunction (with a pair of subexpressions), and use that everywhere we traverse a constraint expression.

lib/Sema/SemaTemplate.cpp

4256–4264

isSatisfied only looks at whether the expression is value-dependent; if it's instantiation-dependent but not value-dependent, then you won't have computed whether it's satisfied but will still permit access to that value. We should be checking whether the arguments are dependent here, not whether they're value-dependent. (And since you need to compute that here anyway, I think it'd make more sense to pass in an "is value-dependent" flag to ConstraintSpecializationExpr::Create.)

lib/Sema/SemaTemplateInstantiate.cpp

522

Please delete this empty else block (I assume this is left behind from prior changes).

test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

8–11

These tests at least theoretically depend on the target; please add a -triple x86_64-linux-gnu or whatever to this file so that this test doesn't fail on targets where sizeof(int) is not 4. (I'm not sure we have any such targets right now, but it's not guaranteed.)

[saar.raz]

Address CR comments by rsmith

[saar.raz]

lib/Sema/SemaConcept.cpp

51–68

Atomic constraints will have bool types anyway, can this really happen?

72–74

Since this part will be changed in later patches, I'd like to delay this fix to a later patch for now.

lib/Sema/SemaTemplateInstantiate.cpp

697–699

In any case, I'm in favor of pushing this to a separate patch (after we have diagnostics for requires exprs, which would suffer from a similar problem)

[rsmith]

Only minor comments, let me know if you'd like me to take another look after addressing these, or just go ahead and commit. Thanks!

clang/lib/AST/ExprCXX.cpp

1676–1677 ↗

(On Diff #224789)

Consider passing IsSatisfied and IsValueDependent as a single Optional<bool> (with None meaning value-dependent).

1719–1720 ↗

(On Diff #224789)

Please add

assert((!isValueDependent() || isInstantiationDependent()) && "should not be value-dependent");

or similar. If we marked the expression as being value-dependent but it's not instantiation-dependent, things would go badly wrong later (we wouldn't substitute into it during instantiation, for example).

clang/lib/Sema/SemaConcept.cpp

39 ↗

(On Diff #224789)

No need for getNonReferenceType() here; expressions never have reference type.

40–41 ↗

(On Diff #224789)

I think it'd be a bit better to remove the getUnqualifiedType() call and use hasSameUnqualifiedType instead. That way the diagnostic below will preserve the cv-qualifications (along with any type sugar that you would need to remove to remove the qualifiers).

clang/lib/Serialization/ASTReaderStmt.cpp

737 ↗

(On Diff #224789)

Please can you add some basic test coverage for the serialization code?

clang/test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

1 ↗

(On Diff #224789)

Can we remove -fconcepts-ts from the RUN line?

include/clang/AST/ExprCXX.h

 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_AST_EXPRCXX_H
 #define LLVM_CLANG_AST_EXPRCXX_H
 
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/OperationKinds.h"
 #include "clang/AST/Stmt.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/UnresolvedSet.h"

 namespace clang {
 
 class ASTContext;
 class DeclAccessPair;
 class IdentifierInfo;
 class LambdaCapture;
 class NonTypeTemplateParmDecl;
 class TemplateParameterList;
+class Sema;
 
 //===--------------------------------------------------------------------===//
 // C++ Expressions.
 //===--------------------------------------------------------------------===//
 
 /// A call to an overloaded operator written using operator
 /// syntax.
 ///
static bool classof(const Stmt *T) {
     return T->getStmtClass() == CoyieldExprClass;
   }
 };
 
 /// Represents a C++2a __builtin_bit_cast(T, v) expression. Used to implement
 /// std::bit_cast. These can sometimes be evaluated as part of a constant
 /// expression, but otherwise CodeGen to a simple memcpy in general.
 class BuiltinBitCastExpr final
     : public ExplicitCastExpr,
       private llvm::TrailingObjects<BuiltinBitCastExpr, CXXBaseSpecifier *> {
   friend class ASTStmtReader;
   friend class CastExpr;
   friend class TrailingObjects;
 
   SourceLocation KWLoc;
   SourceLocation RParenLoc;
 
 public:
   BuiltinBitCastExpr(QualType T, ExprValueKind VK, CastKind CK, Expr *SrcExpr,
                      TypeSourceInfo *DstType, SourceLocation KWLoc,
                      SourceLocation RParenLoc)
       : ExplicitCastExpr(BuiltinBitCastExprClass, T, VK, CK, SrcExpr, 0,
                          DstType),
         KWLoc(KWLoc), RParenLoc(RParenLoc) {}
 
   SourceLocation getBeginLoc() const LLVM_READONLY { return KWLoc; }
   SourceLocation getEndLoc() const LLVM_READONLY { return RParenLoc; }
 
   static bool classof(const Stmt *T) {
     return T->getStmtClass() == BuiltinBitCastExprClass;
   }
 };
 
+/// \brief Represents the specialization of a concept - evaluates to a prvalue
+/// of type bool.
+///
+/// According to C++2a [expr.prim.id]p3 an id-expression that denotes the
+/// specialization of a concept results in a prvalue of type bool.
+class ConceptSpecializationExpr final : public Expr,
+      private llvm::TrailingObjects<ConceptSpecializationExpr,
+                                    TemplateArgument> {
+  friend class ASTStmtReader;
+  friend TrailingObjects;
+
+  // \brief The optional nested name specifier used when naming the concept.
+  NestedNameSpecifierLoc NestedNameSpec;
+
+  /// \brief The location of the template keyword, if specified when naming the
+  /// concept.
+  SourceLocation TemplateKWLoc;
+
+  /// \brief The location of the concept name in the expression.
+  SourceLocation ConceptNameLoc;
+
+  /// \brief The declaration found by name lookup when the expression was
+  /// created.
+  /// Can differ from NamedConcept when, for example, the concept was found
+  /// through a UsingShadowDecl.
+  NamedDecl *FoundDecl;
+
+  /// \brief The concept named, and whether or not the concept with the given
+  /// arguments was satisfied when the expression was created.
+  /// If any of the template arguments are dependent (this expr would then be
+  /// isValueDependent()), this bit is to be ignored.
+  llvm::PointerIntPair<ConceptDecl *, 1, bool> NamedConcept;
+
+  /// \brief The template argument list source info used to specialize the
+  /// concept.
+  const ASTTemplateArgumentListInfo *ArgsAsWritten = nullptr;
+
+  /// \brief The number of template arguments in the tail-allocated list of
+  /// converted template arguments.
+  unsigned NumTemplateArgs;
+
+  ConceptSpecializationExpr(ASTContext &C, NestedNameSpecifierLoc NNS,
+                            SourceLocation TemplateKWLoc,
+                            SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
+                            ConceptDecl *NamedConcept,
+                            const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                            ArrayRef<TemplateArgument> ConvertedArgs,
+                            bool IsSatisfied);
+
+  ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
+
+public:
+
+  static ConceptSpecializationExpr *
+  Create(ASTContext &C, NestedNameSpecifierLoc NNS,
+         SourceLocation TemplateKWLoc, SourceLocation ConceptNameLoc,
+         NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
+         const ASTTemplateArgumentListInfo *ArgsAsWritten,
+         ArrayRef<TemplateArgument> ConvertedArgs, bool IsSatisfied);
+
+  static ConceptSpecializationExpr *
+  Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
+
+  const NestedNameSpecifierLoc &getNestedNameSpecifierLoc() const {
+    return NestedNameSpec;
+  }
+
+  NamedDecl *getFoundDecl() const {
+    return FoundDecl;
+  }
+
+  ConceptDecl *getNamedConcept() const {
+    return NamedConcept.getPointer();
+  }
+
+  ArrayRef<TemplateArgument> getTemplateArguments() const {
+    return ArrayRef<TemplateArgument>(getTrailingObjects<TemplateArgument>(),
+                                      NumTemplateArgs);
+  }
+
+  const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
+    return ArgsAsWritten;
+  }
+
+  /// \brief Set new template arguments for this concept specialization.
+  void setTemplateArguments(const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                            ArrayRef<TemplateArgument> Converted);
+
+  /// \brief Whether or not the concept with the given arguments was satisfied
+  /// when the expression was created. This method assumes that the expression
+  /// is not dependent!
+  bool isSatisfied() const {
+    assert(!isValueDependent()
+           && "isSatisfied called on a dependent ConceptSpecializationExpr");
+    return NamedConcept.getInt();
+  }
+
+  SourceLocation getConceptNameLoc() const { return ConceptNameLoc; }
+
+  SourceLocation getTemplateKWLoc() const { return TemplateKWLoc; }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == ConceptSpecializationExprClass;
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY { return ConceptNameLoc; }
+  SourceLocation getEndLoc() const LLVM_READONLY {
+    return ArgsAsWritten->RAngleLoc;
+  }
+
+  // Iterators
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+};
+
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_EXPRCXX_H

include/clang/AST/RecursiveASTVisitor.h

 })
 DEF_TRAVERSE_STMT(CoyieldExpr, {
   if (!getDerived().shouldVisitImplicitCode()) {
     TRY_TO_TRAVERSE_OR_ENQUEUE_STMT(S->getOperand());
     ShouldVisitChildren = false;
   }
 })
 
+DEF_TRAVERSE_STMT(ConceptSpecializationExpr, {
+  TRY_TO(TraverseTemplateArgumentLocsHelper(
+          S->getTemplateArgsAsWritten()->getTemplateArgs(),
+          S->getTemplateArgsAsWritten()->NumTemplateArgs));
+})
+
 // These literals (all of them) do not need any action.
 DEF_TRAVERSE_STMT(IntegerLiteral, {})
 DEF_TRAVERSE_STMT(FixedPointLiteral, {})
 DEF_TRAVERSE_STMT(CharacterLiteral, {})
 DEF_TRAVERSE_STMT(FloatingLiteral, {})
 DEF_TRAVERSE_STMT(ImaginaryLiteral, {})
 DEF_TRAVERSE_STMT(StringLiteral, {})
 DEF_TRAVERSE_STMT(ObjCStringLiteral, {})

include/clang/Basic/DiagnosticSemaKinds.td

def note_non_literal_lambda : Note<
   "lambda closure types are non-literal types before C++17">;
 def warn_private_extern : Warning<
   "use of __private_extern__ on a declaration may not produce external symbol "
   "private to the linkage unit and is deprecated">, InGroup<PrivateExtern>;
 def note_private_extern : Note<
   "use __attribute__((visibility(\"hidden\"))) attribute instead">;
 
 // C++ Concepts
-def err_concept_initialized_with_non_bool_type : Error<
-  "constraint expression must be of type 'bool' but is of type %0">;
 def err_concept_decls_may_only_appear_in_global_namespace_scope : Error<
   "concept declarations may only appear in global or namespace scope">;
 def err_concept_no_parameters : Error<
   "concept template parameter list must have at least one parameter; explicit "
   "specialization of concepts is not allowed">;
 def err_concept_extra_headers : Error<
   "extraneous template parameter list in concept definition">;
 def err_concept_no_associated_constraints : Error<
   "concept cannot have associated constraints">;
 def err_concept_not_implemented : Error<
   "sorry, unimplemented concepts feature %0 used">;
+def err_non_constant_constraint_expression : Error<
+  "substitution into constraint expression resulted in a non-constant "
+  "expression">;
+def err_non_bool_atomic_constraint : Error<
+  "atomic constraint must be of type 'bool' (found %0)">;
 
 def err_template_different_associated_constraints : Error<
   "associated constraints differ in template redeclaration">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
   "'%0' type specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;

 def note_deduced_template_arg_substitution_here : Note<
   "during template argument deduction for %select{class|variable}0 template "
   "%select{partial specialization |}1%2 %3">;
 def note_prior_template_arg_substitution : Note<
   "while substituting prior template arguments into %select{non-type|template}0"
   " template parameter%1 %2">;
 def note_template_default_arg_checking : Note<
   "while checking a default template argument used here">;
+def note_concept_specialization_here : Note<
+  "while checking the satisfaction of concept '%0' requested here">;
+def note_constraint_substitution_here : Note<
+  "while substituting template arguments into constraint expression here">;
 def note_instantiation_contexts_suppressed : Note<
   "(skipping %0 context%s0 in backtrace; use -ftemplate-backtrace-limit=0 to "
   "see all)">;
 
 def err_field_instantiates_to_function : Error<
   "data member instantiated with function type %0">;
 def err_variable_instantiates_to_function : Error<
   "%select{variable|static data member}0 instantiated with function type %1">;

include/clang/Basic/StmtNodes.td

 def CXXFoldExpr : DStmt<Expr>;
 
 // C++ Coroutines TS expressions
 def CoroutineSuspendExpr : DStmt<Expr, 1>;
 def CoawaitExpr : DStmt<CoroutineSuspendExpr>;
 def DependentCoawaitExpr : DStmt<Expr>;
 def CoyieldExpr : DStmt<CoroutineSuspendExpr>;
 
+// C++2a Concepts expressions
+def ConceptSpecializationExpr : DStmt<Expr>;
+
 // Obj-C Expressions.
 def ObjCStringLiteral : DStmt<Expr>;
 def ObjCBoxedExpr : DStmt<Expr>;
 def ObjCArrayLiteral : DStmt<Expr>;
 def ObjCDictionaryLiteral : DStmt<Expr>;
 def ObjCEncodeExpr : DStmt<Expr>;
 def ObjCMessageExpr : DStmt<Expr>;
 def ObjCSelectorExpr : DStmt<Expr>;

include/clang/Sema/Sema.h

   void DefineImplicitLambdaToBlockPointerConversion(SourceLocation CurrentLoc,
                                                     CXXConversionDecl *Conv);
 
   ExprResult BuildBlockForLambdaConversion(SourceLocation CurrentLocation,
                                            SourceLocation ConvLocation,
                                            CXXConversionDecl *Conv,
                                            Expr *Src);
 
+  /// Check whether the given expression is a valid constraint expression.
+  /// A diagnostic is emitted if it is not, and false is returned.
+  bool CheckConstraintExpression(Expr *CE);
+
+  bool CalculateConstraintSatisfaction(ConceptDecl *NamedConcept,
+                                         MultiLevelTemplateArgumentList &MLTAL,
+                                         Expr *ConstraintExpr,
+                                         bool &IsSatisfied);
+
     // ParseObjCStringLiteral - Parse Objective-C string literals.
   ExprResult ParseObjCStringLiteral(SourceLocation *AtLocs,
                                     ArrayRef<Expr *> Strings);
 
   ExprResult BuildObjCStringLiteral(SourceLocation AtLoc, StringLiteral *S);
 
   /// BuildObjCNumericLiteral - builds an ObjCBoxedExpr AST node for the
   /// numeric literal expression. Type of the expression will be "NSNumber *"
   /// or "id" if NSNumber is unavailable.

   ExprResult CheckVarTemplateId(const CXXScopeSpec &SS,
                                 const DeclarationNameInfo &NameInfo,
                                 VarTemplateDecl *Template,
                                 SourceLocation TemplateLoc,
                                 const TemplateArgumentListInfo *TemplateArgs);
 
   ExprResult
   CheckConceptTemplateId(const CXXScopeSpec &SS,
-                         const DeclarationNameInfo &NameInfo,
-                         ConceptDecl *Template,
-                         SourceLocation TemplateLoc,
+                         SourceLocation TemplateKWLoc,
+                         SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
+                         ConceptDecl *NamedConcept,
                          const TemplateArgumentListInfo *TemplateArgs);
 
   void diagnoseMissingTemplateArguments(TemplateName Name, SourceLocation Loc);
 
   ExprResult BuildTemplateIdExpr(const CXXScopeSpec &SS,
                                  SourceLocation TemplateKWLoc,
                                  LookupResult &R,
                                  bool RequiresADL,

   /// \returns true if an error occurred (e.g., because the parameter packs
   /// are to be instantiated with arguments of different lengths), false
   /// otherwise. If false, \c ShouldExpand (and possibly \c NumExpansions)
   /// must be set.
   bool CheckParameterPacksForExpansion(SourceLocation EllipsisLoc,
                                        SourceRange PatternRange,
                              ArrayRef<UnexpandedParameterPack> Unexpanded,
                              const MultiLevelTemplateArgumentList &TemplateArgs,
                                        bool &ShouldExpand,
                                        bool &RetainExpansion,
                                        Optional<unsigned> &NumExpansions);
 
   /// Determine the number of arguments in the given pack expansion
   /// type.
   ///
   /// This routine assumes that the number of arguments in the expansion is
   /// consistent across all of the unexpanded parameter packs in its pattern.
enum SynthesisKind {
 
       /// We are declaring an implicit special member function.
       DeclaringSpecialMember,
 
       /// We are defining a synthesized function (such as a defaulted special
       /// member).
       DefiningSynthesizedFunction,
 
+      // We are checking the constraints associated with a constrained entity or
+      // the constraint expression of a concept.
+      ConstraintsCheck,
+
+      // We are substituting template arguments into a constraint expression.
+      ConstraintSubstitution,
+
       /// Added for Template instantiation observation.
       /// Memoization means we are _not_ instantiating a template because
       /// it is already instantiated (but we entered a context where we
       /// would have had to if it was not already instantiated).
       Memoization
     } Kind;
 
     /// Was the enclosing context a non-instantiation SFINAE context?
struct InstantiatingTemplate {
     /// Note that we are checking the default template argument
     /// against the template parameter for a given template-id.
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
                           TemplateDecl *Template,
                           NamedDecl *Param,
                           ArrayRef<TemplateArgument> TemplateArgs,
                           SourceRange InstantiationRange);
 
+    struct ConstraintsCheck {};
+    /// \brief Note that we are checking the constraints associated with some
+    /// constrained entity (a concept declaration or a template with associated
+    /// constraints).
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          ConstraintsCheck, TemplateDecl *Template,
+                          ArrayRef<TemplateArgument> TemplateArgs,
+                          SourceRange InstantiationRange);
+
+    struct ConstraintSubstitution {};
+    /// \brief Note that we are checking a constraint expression associated
+    /// with a template declaration or as part of the satisfaction check of a
+    /// concept.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          ConstraintSubstitution, TemplateDecl *Template,
+                          sema::TemplateDeductionInfo &DeductionInfo,
+                          SourceRange InstantiationRange);
 
     /// Note that we have finished instantiating this template.
     void Clear();
 
     ~InstantiatingTemplate() { Clear(); }
 
     /// Determines whether we have exceeded the maximum
     /// recursive template instantiations.

include/clang/Serialization/ASTBitCodes.h

enum StmtCode {
 
       EXPR_PACK_EXPANSION,        // PackExpansionExpr
       EXPR_SIZEOF_PACK,           // SizeOfPackExpr
       EXPR_SUBST_NON_TYPE_TEMPLATE_PARM, // SubstNonTypeTemplateParmExpr
       EXPR_SUBST_NON_TYPE_TEMPLATE_PARM_PACK,// SubstNonTypeTemplateParmPackExpr
       EXPR_FUNCTION_PARM_PACK,    // FunctionParmPackExpr
       EXPR_MATERIALIZE_TEMPORARY, // MaterializeTemporaryExpr
       EXPR_CXX_FOLD,              // CXXFoldExpr
+      EXPR_CONCEPT_SPECIALIZATION,// ConceptSpecializationExpr
 
       // CUDA
       EXPR_CUDA_KERNEL_CALL,       // CUDAKernelCallExpr
 
       // OpenCL
       EXPR_ASTYPE,                 // AsTypeExpr
 
       // Microsoft

lib/AST/Expr.cpp

bool Expr::HasSideEffects(const ASTContext &Ctx,
   case SizeOfPackExprClass:
   case ObjCStringLiteralClass:
   case ObjCEncodeExprClass:
   case ObjCBoolLiteralExprClass:
   case ObjCAvailabilityCheckExprClass:
   case CXXUuidofExprClass:
   case OpaqueValueExprClass:
   case SourceLocExprClass:
+  case ConceptSpecializationExprClass:
     // These never have a side-effect.
     return false;
 
   case ConstantExprClass:
     // FIXME: Move this into the "return false;" block above.
     return cast<ConstantExpr>(this)->getSubExpr()->HasSideEffects(
         Ctx, IncludePossibleEffects);
 

lib/AST/ExprCXX.cpp

 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/LLVM.h"
 #include "clang/Basic/OperatorKinds.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/Specifiers.h"
+#include "clang/Sema/Template.h"
+#include "clang/Sema/SemaDiagnostic.h"
+#include "clang/Sema/Sema.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <cstddef>
 #include <cstring>
 #include <memory>
 
TypeTraitExpr *TypeTraitExpr::CreateDeserialized(const ASTContext &C,
                                                  unsigned NumArgs) {
   void *Mem = C.Allocate(totalSizeToAlloc<TypeSourceInfo *>(NumArgs));
   return new (Mem) TypeTraitExpr(EmptyShell());
 }
 
 CUDAKernelCallExpr::CUDAKernelCallExpr(Expr *Fn, CallExpr *Config,
                                        ArrayRef<Expr *> Args, QualType Ty,
                                        ExprValueKind VK, SourceLocation RP,
                                        unsigned MinNumArgs)
     : CallExpr(CUDAKernelCallExprClass, Fn, /*PreArgs=*/Config, Args, Ty, VK,
                RP, MinNumArgs, NotADL) {}
 
 CUDAKernelCallExpr::CUDAKernelCallExpr(unsigned NumArgs, EmptyShell Empty)
     : CallExpr(CUDAKernelCallExprClass, /*NumPreArgs=*/END_PREARG, NumArgs,
                Empty) {}
 
 CUDAKernelCallExpr *
 CUDAKernelCallExpr::Create(const ASTContext &Ctx, Expr *Fn, CallExpr *Config,
                            ArrayRef<Expr *> Args, QualType Ty, ExprValueKind VK,
                            SourceLocation RP, unsigned MinNumArgs) {
   // Allocate storage for the trailing objects of CallExpr.
   unsigned NumArgs = std::max<unsigned>(Args.size(), MinNumArgs);
   unsigned SizeOfTrailingObjects =
       CallExpr::sizeOfTrailingObjects(/*NumPreArgs=*/END_PREARG, NumArgs);
   void *Mem = Ctx.Allocate(sizeof(CUDAKernelCallExpr) + SizeOfTrailingObjects,
                            alignof(CUDAKernelCallExpr));
   return new (Mem) CUDAKernelCallExpr(Fn, Config, Args, Ty, VK, RP, MinNumArgs);
 }
 
 CUDAKernelCallExpr *CUDAKernelCallExpr::CreateEmpty(const ASTContext &Ctx,
                                                     unsigned NumArgs,
                                                     EmptyShell Empty) {
   // Allocate storage for the trailing objects of CallExpr.
   unsigned SizeOfTrailingObjects =
       CallExpr::sizeOfTrailingObjects(/*NumPreArgs=*/END_PREARG, NumArgs);
   void *Mem = Ctx.Allocate(sizeof(CUDAKernelCallExpr) + SizeOfTrailingObjects,
                            alignof(CUDAKernelCallExpr));
   return new (Mem) CUDAKernelCallExpr(NumArgs, Empty);
 }
+
+ConceptSpecializationExpr::ConceptSpecializationExpr(ASTContext &C,
+    NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
+    SourceLocation ConceptNameLoc, NamedDecl *FoundDecl,
+    ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
+    ArrayRef<TemplateArgument> ConvertedArgs, bool IsSatisfied)
+    : Expr(ConceptSpecializationExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
+           /*TypeDependent=*/false,
+           // All the flags below are set in setTemplateArguments.
+           /*ValueDependent=*/false, /*InstantiationDependent=*/false,
+           /*ContainsUnexpandedParameterPacks=*/false),
+      NestedNameSpec(NNS), TemplateKWLoc(TemplateKWLoc),
+      ConceptNameLoc(ConceptNameLoc), FoundDecl(FoundDecl),
+      NamedConcept(NamedConcept, IsSatisfied),
+      NumTemplateArgs(ConvertedArgs.size()) {
+  setTemplateArguments(ArgsAsWritten, ConvertedArgs);
+}
+
+ConceptSpecializationExpr::ConceptSpecializationExpr(EmptyShell Empty,
+    unsigned NumTemplateArgs)
+    : Expr(ConceptSpecializationExprClass, Empty),
+      NumTemplateArgs(NumTemplateArgs) { }
+
+void ConceptSpecializationExpr::setTemplateArguments(
+    const ASTTemplateArgumentListInfo *ArgsAsWritten,
+    ArrayRef<TemplateArgument> Converted) {
+  assert(Converted.size() == NumTemplateArgs);
+  assert(!this->ArgsAsWritten && "setTemplateArguments can only be used once");
+  this->ArgsAsWritten = ArgsAsWritten;
+  std::uninitialized_copy(Converted.begin(), Converted.end(),
+                          getTrailingObjects<TemplateArgument>());
+  bool IsDependent = false;
+  bool IsInstantiationDependent = false;
+  bool ContainsUnexpandedParameterPack = false;
+  for (const TemplateArgumentLoc& LocInfo : ArgsAsWritten->arguments()) {
+    if (LocInfo.getArgument().isDependent()) {
+      IsDependent = true;
+      if (ContainsUnexpandedParameterPack && IsInstantiationDependent)
+        break;
+    }
+    if (LocInfo.getArgument().isInstantiationDependent()) {
+      IsInstantiationDependent = true;
+      if (ContainsUnexpandedParameterPack && IsDependent)
+        break;
+    }
+    if (LocInfo.getArgument().containsUnexpandedParameterPack()) {
+      ContainsUnexpandedParameterPack = true;
+      if (IsDependent && IsInstantiationDependent)
+        break;
+    }
+  }
+  setValueDependent(IsDependent);
+  setInstantiationDependent(IsInstantiationDependent);
+  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
+}
+
+ConceptSpecializationExpr *
+ConceptSpecializationExpr::Create(ASTContext &C, NestedNameSpecifierLoc NNS,
+                                  SourceLocation TemplateKWLoc,
+                                  SourceLocation ConceptNameLoc,
+                                  NamedDecl *FoundDecl,
+                                  ConceptDecl *NamedConcept,
+                               const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                                  ArrayRef<TemplateArgument> ConvertedArgs,
+                                  bool IsSatisfied) {
+  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
+                                ConvertedArgs.size()));
+  return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
+                                                ConceptNameLoc, FoundDecl,
+                                                NamedConcept, ArgsAsWritten,
+                                                ConvertedArgs, IsSatisfied);
+}
+
+ConceptSpecializationExpr *
+ConceptSpecializationExpr::Create(ASTContext &C, EmptyShell Empty,
+                                  unsigned NumTemplateArgs) {
+  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
+                                NumTemplateArgs));
+  return new (Buffer) ConceptSpecializationExpr(Empty, NumTemplateArgs);
+}
 No newline at end of file

lib/AST/ExprClassification.cpp

#include "clang/AST/StmtNodes.inc"
   case Expr::ObjCIndirectCopyRestoreExprClass:
   case Expr::AtomicExprClass:
   case Expr::CXXFoldExprClass:
   case Expr::ArrayInitLoopExprClass:
   case Expr::ArrayInitIndexExprClass:
   case Expr::NoInitExprClass:
   case Expr::DesignatedInitUpdateExprClass:
   case Expr::SourceLocExprClass:
+  case Expr::ConceptSpecializationExprClass:
     return Cl::CL_PRValue;
 
   case Expr::ConstantExprClass:
     return ClassifyInternal(Ctx, cast<ConstantExpr>(E)->getSubExpr());
 
     // Next come the complicated cases.
   case Expr::SubstNonTypeTemplateParmExprClass:
     return ClassifyInternal(Ctx,

lib/AST/ExprConstant.cpp

public:
   }
 
   bool VisitUnaryReal(const UnaryOperator *E);
   bool VisitUnaryImag(const UnaryOperator *E);
 
   bool VisitCXXNoexceptExpr(const CXXNoexceptExpr *E);
   bool VisitSizeOfPackExpr(const SizeOfPackExpr *E);
   bool VisitSourceLocExpr(const SourceLocExpr *E);
+  bool VisitConceptSpecializationExpr(const ConceptSpecializationExpr *E);
   // FIXME: Missing: array subscript of vector, member of vector
 };
 
 class FixedPointExprEvaluator
     : public ExprEvaluatorBase<FixedPointExprEvaluator> {
   APValue &Result;
 
  public:

 bool IntExprEvaluator::VisitSizeOfPackExpr(const SizeOfPackExpr *E) {
   return Success(E->getPackLength(), E);
 }
 
 bool IntExprEvaluator::VisitCXXNoexceptExpr(const CXXNoexceptExpr *E) {
   return Success(E->getValue(), E);
 }
 
+bool IntExprEvaluator::VisitConceptSpecializationExpr(
+       const ConceptSpecializationExpr *E) {
+  return Success(E->isSatisfied(), E);
+}
+
+
 bool FixedPointExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
   switch (E->getOpcode()) {
     default:
       // Invalid unary operators
       return Error(E);
     case UO_Plus:
       // The result is just the value.
       return Visit(E->getSubExpr());
case Expr::GenericSelectionExprClass:
     return CheckICE(cast<GenericSelectionExpr>(E)->getResultExpr(), Ctx);
   case Expr::IntegerLiteralClass:
   case Expr::FixedPointLiteralClass:
   case Expr::CharacterLiteralClass:
   case Expr::ObjCBoolLiteralExprClass:
   case Expr::CXXBoolLiteralExprClass:
   case Expr::CXXScalarValueInitExprClass:
   case Expr::TypeTraitExprClass:
+  case Expr::ConceptSpecializationExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::CXXNoexceptExprClass:
     return NoDiag();
   case Expr::CallExprClass:
   case Expr::CXXOperatorCallExprClass: {
     // C99 6.6/3 allows function calls within unevaluated subexpressions of
     // constant expressions, but they can never be ICEs because an ICE cannot

lib/AST/ItaniumMangle.cpp

#include "clang/AST/StmtNodes.inc"
   case Expr::ObjCIndirectCopyRestoreExprClass:
   case Expr::ObjCAvailabilityCheckExprClass:
   case Expr::OffsetOfExprClass:
   case Expr::PredefinedExprClass:
   case Expr::ShuffleVectorExprClass:
   case Expr::ConvertVectorExprClass:
   case Expr::StmtExprClass:
   case Expr::TypeTraitExprClass:
+  case Expr::ConceptSpecializationExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::VAArgExprClass:
   case Expr::CUDAKernelCallExprClass:
   case Expr::AsTypeExprClass:
   case Expr::PseudoObjectExprClass:
   case Expr::AtomicExprClass:
   case Expr::SourceLocExprClass:

lib/AST/StmtPrinter.cpp

void StmtPrinter::VisitCXXFoldExpr(CXXFoldExpr *E) {
   OS << "...";
   if (E->getRHS()) {
     OS << " " << BinaryOperator::getOpcodeStr(E->getOperator()) << " ";
     PrintExpr(E->getRHS());
   }
   OS << ")";
 }
 
+void StmtPrinter::VisitConceptSpecializationExpr(ConceptSpecializationExpr *E) {
+  NestedNameSpecifierLoc NNS = E->getNestedNameSpecifierLoc();
+  if (NNS)
+    NNS.getNestedNameSpecifier()->print(OS, Policy);
+  if (E->getTemplateKWLoc().isValid())
+    OS << "template ";
+  OS << E->getFoundDecl()->getName();
+  printTemplateArgumentList(OS, E->getTemplateArgsAsWritten()->arguments(),
+                            Policy);
+}
+
 // C++ Coroutines TS
 
 void StmtPrinter::VisitCoroutineBodyStmt(CoroutineBodyStmt *S) {
   Visit(S->getBody());
 }
 
 void StmtPrinter::VisitCoreturnStmt(CoreturnStmt *S) {
   OS << "co_return";

lib/AST/StmtProfile.cpp

if (const OpaqueValueExpr *OVE = dyn_cast<OpaqueValueExpr>(*i))
       Visit(OVE->getSourceExpr());
 }
 
 void StmtProfiler::VisitAtomicExpr(const AtomicExpr *S) {
   VisitExpr(S);
   ID.AddInteger(S->getOp());
 }
 
+void StmtProfiler::VisitConceptSpecializationExpr(
+                                           const ConceptSpecializationExpr *S) {
+  VisitExpr(S);
+  VisitDecl(S->getNamedConcept());
+  VisitTemplateArguments(S->getTemplateArgsAsWritten()->getTemplateArgs(),
+                         S->getTemplateArgsAsWritten()->NumTemplateArgs);
+}
+
 static Stmt::StmtClass DecodeOperatorCall(const CXXOperatorCallExpr *S,
                                           UnaryOperatorKind &UnaryOp,
                                           BinaryOperatorKind &BinaryOp) {
   switch (S->getOperator()) {
   case OO_None:
   case OO_New:
   case OO_Delete:
   case OO_Array_New:

lib/CodeGen/CGExprScalar.cpp

Value *VisitCXXDeleteExpr(const CXXDeleteExpr *E) {
     CGF.EmitCXXDeleteExpr(E);
     return nullptr;
   }
 
   Value *VisitTypeTraitExpr(const TypeTraitExpr *E) {
     return llvm::ConstantInt::get(ConvertType(E->getType()), E->getValue());
   }
 
+  Value *VisitConceptSpecializationExpr(const ConceptSpecializationExpr *E) {
+    return Builder.getInt1(E->isSatisfied());
+  }
+
   Value *VisitArrayTypeTraitExpr(const ArrayTypeTraitExpr *E) {
     return llvm::ConstantInt::get(Builder.getInt32Ty(), E->getValue());
   }
 
   Value *VisitExpressionTraitExpr(const ExpressionTraitExpr *E) {
     return llvm::ConstantInt::get(Builder.getInt1Ty(), E->getValue());
   }
 

lib/Frontend/FrontendActions.cpp

static std::string toString(CodeSynthesisContext::SynthesisKind Kind) {
     case CodeSynthesisContext::ExceptionSpecInstantiation:
       return "ExceptionSpecInstantiation";
     case CodeSynthesisContext::DeclaringSpecialMember:
       return "DeclaringSpecialMember";
     case CodeSynthesisContext::DefiningSynthesizedFunction:
       return "DefiningSynthesizedFunction";
     case CodeSynthesisContext::Memoization:
       return "Memoization";
+    case CodeSynthesisContext::ConstraintsCheck:
+      return "ConstraintsCheck";
+    case CodeSynthesisContext::ConstraintSubstitution:
+      return "ConstraintSubstitution";
     }
     return "";
   }
 
   template <bool BeginInstantiation>
   static void displayTemplightEntry(llvm::raw_ostream &Out, const Sema &TheSema,
                                     const CodeSynthesisContext &Inst) {
     std::string YAML;

lib/Parse/ParseExpr.cpp

ExprResult Parser::ParseCaseExpression(SourceLocation CaseLoc) {
   ExprResult LHS(ParseCastExpression(false, false, NotTypeCast));
   ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::Conditional));
   return Actions.ActOnCaseExpr(CaseLoc, Res);
 }
 
 /// Parse a constraint-expression.
 ///
 /// \verbatim
-///       constraint-expression: [Concepts TS temp.constr.decl p1]
+///       constraint-expression: C++2a[temp.constr.decl]p1
 ///         logical-or-expression
 /// \endverbatim
 ExprResult Parser::ParseConstraintExpression() {
-  // FIXME: this may erroneously consume a function-body as the braced
-  // initializer list of a compound literal
-  //
-  // FIXME: this may erroneously consume a parenthesized rvalue reference
-  // declarator as a parenthesized address-of-label expression
+  EnterExpressionEvaluationContext ConstantEvaluated(
+      Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
   ExprResult LHS(ParseCastExpression(/*isUnaryExpression=*/false));
   ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::LogicalOr));
-
+  if (Res.isUsable() && !Actions.CheckConstraintExpression(Res.get()))
+    return ExprError();
   return Res;
 }
 
 bool Parser::isNotExpressionStart() {
   tok::TokenKind K = Tok.getKind();
   if (K == tok::l_brace || K == tok::r_brace  ||
       K == tok::kw_for  || K == tok::kw_while ||
       K == tok::kw_if   || K == tok::kw_else  ||
       K == tok::kw_goto || K == tok::kw_try)

lib/Sema/CMakeLists.txt

add_clang_library(clangSema
   ScopeInfo.cpp
   Sema.cpp
   SemaAccess.cpp
   SemaAttr.cpp
   SemaCXXScopeSpec.cpp
   SemaCast.cpp
   SemaChecking.cpp
   SemaCodeComplete.cpp
+  SemaConcept.cpp
   SemaConsumer.cpp
   SemaCoroutine.cpp
   SemaCUDA.cpp
   SemaDecl.cpp
   SemaDeclAttr.cpp
   SemaDeclCXX.cpp
   SemaDeclObjC.cpp
   SemaExceptionSpec.cpp

lib/Sema/SemaConcept.cpp

+//===-- SemaConcept.cpp - Semantic Analysis for Constraints and Concepts --===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file implements semantic analysis for C++ constraints and concepts.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Sema/Sema.h"
+#include "clang/Sema/SemaDiagnostic.h"
+#include "clang/Sema/TemplateDeduction.h"
+#include "clang/Sema/Template.h"
+#include "clang/AST/ExprCXX.h"
+using namespace clang;
+using namespace sema;
+
+bool Sema::CheckConstraintExpression(Expr *ConstraintExpression) {
+  // C++2a [temp.constr.atomic]p1
+  // ..E shall be a constant expression of type bool.
+
+  if (auto *BinOp = dyn_cast<BinaryOperator>(ConstraintExpression))
+    if (BinOp->getOpcode() == BO_LAnd || BinOp->getOpcode() == BO_LOr)
+      return CheckConstraintExpression(BinOp->getLHS()) &&
+          CheckConstraintExpression(BinOp->getRHS());
+
+  // An atomic constraint!
+  if (ConstraintExpression->isTypeDependent())
+    return true;
+
+  QualType Type = ConstraintExpression->IgnoreParenImpCasts()->getType()
+                      .getNonReferenceType().getUnqualifiedType();
+  if (!Context.hasSameType(Type, Context.BoolTy)) {
+    Diag(ConstraintExpression->getExprLoc(),
+         diag::err_non_bool_atomic_constraint) << Type
+        << ConstraintExpression->getSourceRange();
+    return false;
+  }
+  return true;
+}
+
+bool
+Sema::CalculateConstraintSatisfaction(ConceptDecl *NamedConcept,
+                                      MultiLevelTemplateArgumentList &MLTAL,
+                                      Expr *ConstraintExpr,
+                                      bool &IsSatisfied) {
+  if (auto *BO = dyn_cast<BinaryOperator>(ConstraintExpr)) {
+    if (BO->getOpcode() == BO_LAnd) {
+      if (CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getLHS(),
+                                          IsSatisfied))
+        return true;
+      if (!IsSatisfied)
+        return false;
+      return CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getRHS(),
+                                             IsSatisfied);
+    } else if (BO->getOpcode() == BO_LOr) {
+      if (CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getLHS(),
+                                          IsSatisfied))
+        return true;
+      if (IsSatisfied)
+        return false;
+      return CalculateConstraintSatisfaction(NamedConcept, MLTAL, BO->getRHS(),
+                                             IsSatisfied);
+    }
+  } else if (auto *PO = dyn_cast<ParenExpr>(ConstraintExpr))
+    return CalculateConstraintSatisfaction(NamedConcept, MLTAL,
+                                           PO->getSubExpr(), IsSatisfied);
+  else if (auto *C = dyn_cast<ExprWithCleanups>(ConstraintExpr))
+    return CalculateConstraintSatisfaction(NamedConcept, MLTAL, C->getSubExpr(),
+                                           IsSatisfied);
+
+  EnterExpressionEvaluationContext ConstantEvaluated(
+      *this, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+
+  // Atomic constraint - substitute arguments and check satisfaction.
+  ExprResult E;
+  {
+    TemplateDeductionInfo Info(ConstraintExpr->getBeginLoc());
+    InstantiatingTemplate Inst(*this, ConstraintExpr->getBeginLoc(),
+                               InstantiatingTemplate::ConstraintSubstitution{},
+                               NamedConcept, Info,
+                               ConstraintExpr->getSourceRange());
+    if (Inst.isInvalid())
+      return true;
+    // We do not want error diagnostics escaping here.
+    Sema::SFINAETrap Trap(*this);
+
+    E = SubstExpr(ConstraintExpr, MLTAL);
+    if (E.isInvalid() || Trap.hasErrorOccurred()) {
+      // C++2a [temp.constr.atomic]p1
+      //   ...If substitution results in an invalid type or expression, the
+      //   constraint is not satisfied.
+      IsSatisfied = false;
+      return false;
+    }
+  }
+
+  if (!CheckConstraintExpression(E.get()))
+    return true;
+
+  SmallVector<PartialDiagnosticAt, 2> EvaluationDiags;
+  Expr::EvalResult EvalResult;
+  EvalResult.Diag = &EvaluationDiags;
+  if (!E.get()->EvaluateAsRValue(EvalResult, Context)) {
+    // C++2a [temp.constr.atomic]p1
+    //   ...E shall be a constant expression of type bool.
+    Diag(E.get()->getBeginLoc(),
+         diag::err_non_constant_constraint_expression)
+        << E.get()->getSourceRange();
+    for (const PartialDiagnosticAt &PDiag : EvaluationDiags)
+      Diag(PDiag.first, PDiag.second);
+    return true;
+  }
+
+  IsSatisfied = EvalResult.Val.getInt().getBoolValue();
+
+  return false;
+}

lib/Sema/SemaExceptionSpec.cpp

CanThrowResult Sema::canThrow(const Expr *E) {
   case Expr::ObjCEncodeExprClass:
   case Expr::ObjCStringLiteralClass:
   case Expr::ObjCBoolLiteralExprClass:
   case Expr::OpaqueValueExprClass:
   case Expr::PredefinedExprClass:
   case Expr::SizeOfPackExprClass:
   case Expr::StringLiteralClass:
   case Expr::SourceLocExprClass:
+  case Expr::ConceptSpecializationExprClass:
     // These expressions can never throw.
     return CT_Cannot;
 
   case Expr::MSPropertyRefExprClass:
   case Expr::MSPropertySubscriptExprClass:
     llvm_unreachable("Invalid class for expression");
 
 #define STMT(CLASS, PARENT) case Expr::CLASS##Class:

lib/Sema/SemaTemplate.cpp

void Sema::diagnoseMissingTemplateArguments(TemplateName Name,
   if (TemplateDecl *TD = Name.getAsTemplateDecl()) {
     Diag(TD->getLocation(), diag::note_template_decl_here)
       << TD->getTemplateParameters()->getSourceRange();
   }
 }
 
 ExprResult
 Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
-                             const DeclarationNameInfo &NameInfo,
-                             ConceptDecl *Template,
-                             SourceLocation TemplateLoc,
+                             SourceLocation TemplateKWLoc,
+                             SourceLocation ConceptNameLoc,
+                             NamedDecl *FoundDecl,
+                             ConceptDecl *NamedConcept,
                              const TemplateArgumentListInfo *TemplateArgs) {
-  // TODO: Do concept specialization here.
-  Diag(NameInfo.getBeginLoc(), diag::err_concept_not_implemented) <<
-    "concept specialization";
+  assert(NamedConcept && "A concept template id without a template?");
+
+  llvm::SmallVector<TemplateArgument, 4> Converted;
+  if (CheckTemplateArgumentList(NamedConcept, ConceptNameLoc,
+                           const_cast<TemplateArgumentListInfo&>(*TemplateArgs),
+                                /*PartialTemplateArgs=*/false, Converted,
+                                /*UpdateArgsWithConversion=*/false))
+    return ExprError();
+
+  bool IsSatisfied = true;
+  bool IsInstantiationDependent = false;
+  for (TemplateArgument &Arg : Converted) {
+    if (Arg.isInstantiationDependent()) {
+      IsInstantiationDependent = true;
+      break;
+    }
+  }
+  if (!IsInstantiationDependent) {
+    InstantiatingTemplate Inst(*this, ConceptNameLoc,
+        InstantiatingTemplate::ConstraintsCheck{}, NamedConcept, Converted,
+        SourceRange(SS.isSet() ? SS.getBeginLoc() : ConceptNameLoc,
+                    TemplateArgs->getRAngleLoc()));
+    MultiLevelTemplateArgumentList MLTAL;
+    MLTAL.addOuterTemplateArguments(Converted);
+    if (CalculateConstraintSatisfaction(NamedConcept, MLTAL,
+                                        NamedConcept->getConstraintExpr(),
+                                        IsSatisfied))
       return ExprError();
   }
+  return ConceptSpecializationExpr::Create(Context,
+      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc{},
+      TemplateKWLoc, ConceptNameLoc, FoundDecl, NamedConcept,
+      ASTTemplateArgumentListInfo::Create(Context, *TemplateArgs), Converted,
+      IsSatisfied);
+}
 
 ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
                                      SourceLocation TemplateKWLoc,
                                      LookupResult &R,
                                      bool RequiresADL,
                                  const TemplateArgumentListInfo *TemplateArgs) {
   // FIXME: Can we do any checking at this point? I guess we could check the
   // template arguments that we have against the template name, if the template
ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
   // In C++1y, check variable template ids.
   if (R.getAsSingle<VarTemplateDecl>() && !AnyDependentArguments()) {
     return CheckVarTemplateId(SS, R.getLookupNameInfo(),
                               R.getAsSingle<VarTemplateDecl>(),
                               TemplateKWLoc, TemplateArgs);
   }
 
   if (R.getAsSingle<ConceptDecl>() && !AnyDependentArguments()) {
-    return CheckConceptTemplateId(SS, R.getLookupNameInfo(),
-                                  R.getAsSingle<ConceptDecl>(),
-                                  TemplateKWLoc, TemplateArgs);
+    return CheckConceptTemplateId(SS, TemplateKWLoc,
+                                  R.getLookupNameInfo().getBeginLoc(),
+                                  R.getFoundDecl(),
+                                  R.getAsSingle<ConceptDecl>(), TemplateArgs);
   }
 
   // We don't want lookup warnings at this point.
   R.suppressDiagnostics();
 
   UnresolvedLookupExpr *ULE
     = UnresolvedLookupExpr::Create(Context, R.getNamingClass(),
                                    SS.getWithLocInContext(Context),
Decl *Sema::ActOnConceptDefinition(Scope *S,
   if (TemplateParameterLists.front()->size() == 0) {
     Diag(NameLoc, diag::err_concept_no_parameters);
     return nullptr;
   }
 
   ConceptDecl *NewDecl = ConceptDecl::Create(Context, DC, NameLoc, Name,
                                              TemplateParameterLists.front(),
                                              ConstraintExpr);
                                              
-  if (!ConstraintExpr->isTypeDependent() &&
-      ConstraintExpr->getType() != Context.BoolTy) {
-    // C++2a [temp.constr.atomic]p3:
-    // E shall be a constant expression of type bool.
-    // TODO: Do this check for individual atomic constraints
-    // and not the constraint expression. Probably should do it in
-    // ParseConstraintExpression.
-    Diag(ConstraintExpr->getSourceRange().getBegin(),
-        diag::err_concept_initialized_with_non_bool_type)
-      << ConstraintExpr->getType();
-    NewDecl->setInvalidDecl();
-  }
-
   if (NewDecl->getAssociatedConstraints()) {
     // C++2a [temp.concept]p4:
     // A concept shall not have associated constraints.
     // TODO: Make a test once we have actual associated constraints.
     Diag(NameLoc, diag::err_concept_no_associated_constraints);
     NewDecl->setInvalidDecl();
   }
 

lib/Sema/SemaTemplateInstantiate.cpp

bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   switch (Kind) {
   case TemplateInstantiation:
   case ExceptionSpecInstantiation:
   case DefaultTemplateArgumentInstantiation:
   case DefaultFunctionArgumentInstantiation:
   case ExplicitTemplateArgumentSubstitution:
   case DeducedTemplateArgumentSubstitution:
   case PriorTemplateArgumentSubstitution:
+  case ConstraintsCheck:
     return true;
 
   case DefaultTemplateArgumentChecking:
   case DeclaringSpecialMember:
   case DefiningSynthesizedFunction:
   case ExceptionSpecEvaluation:
+  case ConstraintSubstitution:
     return false;
 
   // This function should never be called when Kind's value is Memoization.
   case Memoization:
     break;
   }
 
   llvm_unreachable("Invalid SynthesisKind!");
Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation, TemplateDecl *Template,
     NamedDecl *Param, ArrayRef<TemplateArgument> TemplateArgs,
     SourceRange InstantiationRange)
     : InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::DefaultTemplateArgumentChecking,
           PointOfInstantiation, InstantiationRange, Param, Template,
           TemplateArgs) {}
 
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    ConstraintsCheck, TemplateDecl *Template,
+    ArrayRef<TemplateArgument> TemplateArgs, SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::ConstraintsCheck,
+          PointOfInstantiation, InstantiationRange, Template, nullptr,
+          TemplateArgs) {}
+
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    ConstraintSubstitution, TemplateDecl *Template,
+    sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::ConstraintSubstitution,
+          PointOfInstantiation, InstantiationRange, Template, nullptr,
+          {}, &DeductionInfo) {}
+
 void Sema::pushCodeSynthesisContext(CodeSynthesisContext Ctx) {
   Ctx.SavedInNonInstantiationSFINAEContext = InNonInstantiationSFINAEContext;
   InNonInstantiationSFINAEContext = false;
 
   CodeSynthesisContexts.push_back(Ctx);
 
   if (!Ctx.isInstantiationRecord())
     ++NonInstantiationEntries;
case CodeSynthesisContext::TemplateInstantiation: {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_enum_def_here)
           << ED
           << Active->InstantiationRange;
       } else if (FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_nsdmi_here)
             << FD << Active->InstantiationRange;
-      } else {
+      } else if (auto *AD = dyn_cast<TypeAliasTemplateDecl>(D)) {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_type_alias_instantiation_here)
-          << cast<TypeAliasTemplateDecl>(D)
+          << AD
           << Active->InstantiationRange;
+      } else {
       }
       break;
     }
 
     case CodeSynthesisContext::DefaultTemplateArgumentInstantiation: {
       TemplateDecl *Template = cast<TemplateDecl>(Active->Template);
       SmallVector<char, 128> TemplateArgsStr;
       llvm::raw_svector_ostream OS(TemplateArgsStr);
case CodeSynthesisContext::DefiningSynthesizedFunction: {
                      diag::note_member_synthesized_at)
           << CSM << Context.getTagDeclType(MD->getParent());
       }
       break;
     }
 
     case CodeSynthesisContext::Memoization:
       break;
+    
+    case CodeSynthesisContext::ConstraintsCheck:
+      if (auto *CD = dyn_cast<ConceptDecl>(Active->Entity)) {
+        SmallVector<char, 128> TemplateArgsStr;
+        llvm::raw_svector_ostream OS(TemplateArgsStr);
+        CD->printName(OS);
+        printTemplateArgumentList(OS, Active->template_arguments(),
+                                  getPrintingPolicy());
+        Diags.Report(Active->PointOfInstantiation,
+                     diag::note_concept_specialization_here)
+          << OS.str()
+          << Active->InstantiationRange;
+        break;
+      }
+      // TODO: Concepts - implement this for constrained templates and partial
+      // specializations.
+      llvm_unreachable("only concept constraints are supported right now");
+      break;
+      
+    case CodeSynthesisContext::ConstraintSubstitution:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_constraint_substitution_here)
+          << Active->InstantiationRange;
+      break;
     }
   }
 }
 
 Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
   if (InNonInstantiationSFINAEContext)
     return Optional<TemplateDeductionInfo *>(nullptr);
 
   for (SmallVectorImpl<CodeSynthesisContext>::const_reverse_iterator
          Active = CodeSynthesisContexts.rbegin(),
          ActiveEnd = CodeSynthesisContexts.rend();
        Active != ActiveEnd;
        ++Active)
   {
     switch (Active->Kind) {
     case CodeSynthesisContext::TemplateInstantiation:
       // An instantiation of an alias template may or may not be a SFINAE
       // context, depending on what else is on the stack.
       if (isa<TypeAliasTemplateDecl>(Active->Entity))
         break;
       LLVM_FALLTHROUGH;
     case CodeSynthesisContext::DefaultFunctionArgumentInstantiation:
     case CodeSynthesisContext::ExceptionSpecInstantiation:
+    case CodeSynthesisContext::ConstraintsCheck:
       // This is a template instantiation, so there is no SFINAE.
       return None;
 
     case CodeSynthesisContext::DefaultTemplateArgumentInstantiation:
     case CodeSynthesisContext::PriorTemplateArgumentSubstitution:
     case CodeSynthesisContext::DefaultTemplateArgumentChecking:
       // A default template argument instantiation and substitution into
       // template parameters with arguments for prior parameters may or may
       // not be a SFINAE context; look further up the stack.
       break;
 
     case CodeSynthesisContext::ExplicitTemplateArgumentSubstitution:
     case CodeSynthesisContext::DeducedTemplateArgumentSubstitution:
-      // We're either substitution explicitly-specified template arguments
-      // or deduced template arguments, so SFINAE applies.
+    case CodeSynthesisContext::ConstraintSubstitution:
+      // We're either substituting explicitly-specified template arguments
+      // or deduced template arguments or a constraint expression, so SFINAE
+      // applies.
       assert(Active->DeductionInfo && "Missing deduction info pointer");
       return Active->DeductionInfo;
 
     case CodeSynthesisContext::DeclaringSpecialMember:
     case CodeSynthesisContext::DefiningSynthesizedFunction:
       // This happens in a context unrelated to template instantiation, so
       // there is no SFINAE.
       return None;

lib/Sema/TreeTransform.h

ExprResult RebuildSizeOfPackExpr(SourceLocation OperatorLoc,
                                    SourceLocation PackLoc,
                                    SourceLocation RParenLoc,
                                    Optional<unsigned> Length,
                                    ArrayRef<TemplateArgument> PartialArgs) {
     return SizeOfPackExpr::Create(SemaRef.Context, OperatorLoc, Pack, PackLoc,
                                   RParenLoc, Length, PartialArgs);
   }
 
   /// Build a new expression representing a call to a source location
   ///  builtin.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
   ExprResult RebuildSourceLocExpr(SourceLocExpr::IdentKind Kind,
                                   SourceLocation BuiltinLoc,
                                   SourceLocation RPLoc,
                                   DeclContext *ParentContext) {
     return getSema().BuildSourceLocExpr(Kind, BuiltinLoc, RPLoc, ParentContext);
   }
 
   /// Build a new Objective-C boxed expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildConceptSpecializationExpr(NestedNameSpecifierLoc NNS,
+      SourceLocation TemplateKWLoc, SourceLocation ConceptNameLoc,
+      NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
+      TemplateArgumentListInfo *TALI) {
+    CXXScopeSpec SS;
+    SS.Adopt(NNS);
+    ExprResult Result = getSema().CheckConceptTemplateId(SS, TemplateKWLoc,
+                                                         ConceptNameLoc,
+                                                         FoundDecl,
+                                                         NamedConcept, TALI);
+    if (Result.isInvalid())
+      return ExprError();
+    return Result;
+  }
+
+    /// \brief Build a new Objective-C boxed expression.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
   ExprResult RebuildObjCBoxedExpr(SourceRange SR, Expr *ValueExpr) {
     return getSema().BuildObjCBoxedExpr(SR, ValueExpr);
   }
 
   /// Build a new Objective-C array literal.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
if (!getDerived().AlwaysRebuild() && !ArgChanged)
     return E;
 
   return getDerived().RebuildTypeTrait(E->getTrait(), E->getBeginLoc(), Args,
                                        E->getEndLoc());
 }
 
 template<typename Derived>
 ExprResult
+TreeTransform<Derived>::TransformConceptSpecializationExpr(
+                                                 ConceptSpecializationExpr *E) {
+  const ASTTemplateArgumentListInfo *Old = E->getTemplateArgsAsWritten();
+  TemplateArgumentListInfo TransArgs(Old->LAngleLoc, Old->RAngleLoc);
+  if (getDerived().TransformTemplateArguments(Old->getTemplateArgs(),
+                                              Old->NumTemplateArgs, TransArgs))
+    return ExprError();
+
+  return getDerived().RebuildConceptSpecializationExpr(
+      E->getNestedNameSpecifierLoc(), E->getTemplateKWLoc(),
+      E->getConceptNameLoc(), E->getFoundDecl(), E->getNamedConcept(),
+      &TransArgs);
+}
+
+
+template<typename Derived>
+ExprResult
 TreeTransform<Derived>::TransformArrayTypeTraitExpr(ArrayTypeTraitExpr *E) {
   TypeSourceInfo *T = getDerived().TransformType(E->getQueriedTypeSourceInfo());
   if (!T)
     return ExprError();
 
   if (!getDerived().AlwaysRebuild() &&
       T == E->getQueriedTypeSourceInfo())
     return E;

lib/Serialization/ASTReaderStmt.cpp

if (Record.peekInt() == 0) {
     Record.skipInts(1);
   } else {
     E->setArgument(GetTypeSourceInfo());
   }
   E->setOperatorLoc(ReadSourceLocation());
   E->setRParenLoc(ReadSourceLocation());
 }
 
+void ASTStmtReader::VisitConceptSpecializationExpr(
+        ConceptSpecializationExpr *E) {
+  VisitExpr(E);
+  unsigned NumTemplateArgs = Record.readInt();
+  E->NestedNameSpec = Record.readNestedNameSpecifierLoc();
+  E->TemplateKWLoc = Record.readSourceLocation();
+  E->ConceptNameLoc = Record.readSourceLocation();
+  E->FoundDecl = ReadDeclAs<NamedDecl>();
+  E->NamedConcept.setPointer(ReadDeclAs<ConceptDecl>());
+  const ASTTemplateArgumentListInfo *ArgsAsWritten =
+      Record.readASTTemplateArgumentListInfo();
+  llvm::SmallVector<TemplateArgument, 4> Args;
+  for (unsigned I = 0; I < NumTemplateArgs; ++I)
+    Args.push_back(Record.readTemplateArgument());
+  E->setTemplateArguments(ArgsAsWritten, Args);
+  E->NamedConcept.setInt(Record.readInt() == 1);
+}
+
 void ASTStmtReader::VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
   VisitExpr(E);
   E->setLHS(Record.readSubExpr());
   E->setRHS(Record.readSubExpr());
   E->setRBracketLoc(ReadSourceLocation());
 }
 
 void ASTStmtReader::VisitOMPArraySectionExpr(OMPArraySectionExpr *E) {

 
     case EXPR_COYIELD:
       S = new (Context) CoyieldExpr(Empty);
       break;
 
     case EXPR_DEPENDENT_COAWAIT:
       S = new (Context) DependentCoawaitExpr(Empty);
       break;
+
+    case EXPR_CONCEPT_SPECIALIZATION:
+      unsigned numTemplateArgs = Record[ASTStmtReader::NumExprFields];
+      S = ConceptSpecializationExpr::Create(Context, Empty, numTemplateArgs);
+      break;
+      
     }
 
     // We hit a STMT_STOP, so we're done with this expression.
     if (Finished)
       break;
 
     ++NumStatementsRead;
 

lib/Serialization/ASTWriterStmt.cpp

 void ASTStmtWriter::VisitDependentCoawaitExpr(DependentCoawaitExpr *E) {
   VisitExpr(E);
   Record.AddSourceLocation(E->getKeywordLoc());
   for (Stmt *S : E->children())
     Record.AddStmt(S);
   Code = serialization::EXPR_DEPENDENT_COAWAIT;
 }
 
+void ASTStmtWriter::VisitConceptSpecializationExpr(
+        ConceptSpecializationExpr *E) {
+  VisitExpr(E);
+  ArrayRef<TemplateArgument> TemplateArgs = E->getTemplateArguments();
+  Record.push_back(TemplateArgs.size());
+  Record.AddNestedNameSpecifierLoc(E->getNestedNameSpecifierLoc());
+  Record.AddSourceLocation(E->getTemplateKWLoc());
+  Record.AddSourceLocation(E->getConceptNameLoc());
+  Record.AddDeclRef(E->getFoundDecl());
+  Record.AddDeclRef(E->getNamedConcept());
+  Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
+  for (const TemplateArgument &Arg : TemplateArgs)
+    Record.AddTemplateArgument(Arg);
+  Record.push_back(E->isSatisfied());
+  Code = serialization::EXPR_CONCEPT_SPECIALIZATION;
+}
+
+
 void ASTStmtWriter::VisitCapturedStmt(CapturedStmt *S) {
   VisitStmt(S);
   // NumCaptures
   Record.push_back(std::distance(S->capture_begin(), S->capture_end()));
 
   // CapturedDecl and captured region kind
   Record.AddDeclRef(S->getCapturedDecl());
   Record.push_back(S->getCapturedRegionKind());

lib/StaticAnalyzer/Core/ExprEngine.cpp

switch (S->getStmtClass()) {
     case Stmt::ObjCSelectorExprClass:
     case Stmt::ParenListExprClass:
     case Stmt::ShuffleVectorExprClass:
     case Stmt::ConvertVectorExprClass:
     case Stmt::VAArgExprClass:
     case Stmt::CUDAKernelCallExprClass:
     case Stmt::OpaqueValueExprClass:
     case Stmt::AsTypeExprClass:
+    case Stmt::ConceptSpecializationExprClass:
       // Fall through.
 
     // Cases we intentionally don't evaluate, since they don't need
     // to be explicitly evaluated.
     case Stmt::PredefinedExprClass:
     case Stmt::AddrLabelExprClass:
     case Stmt::AttributedStmtClass:
     case Stmt::IntegerLiteralClass:

test/CXX/concepts-ts/expr/expr.prim/expr.prim.id/p3.cpp

 // RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
 
-template<typename T> concept C = true;
-static_assert(C<int>); // expected-error{{sorry, unimplemented concepts feature concept specialization used}}
+template<typename T> concept C1 = true; // expected-note{{template is declared here}}
+static_assert(C1<int>);
+static_assert(C1);
+// expected-error@-1{{use of concept 'C1' requires template arguments}}
+
+template<typename T> concept C2 = sizeof(T) == 4;
+static_assert(C2<int>);
+static_assert(!C2<long long int>);
+static_assert(C2<char[4]>);
+static_assert(!C2<char[5]>);
+
+template<typename T> concept C3 = sizeof(*T{}) == 4;
+static_assert(C3<int*>);
+static_assert(!C3<long long int>);
+
+struct A {
+  static constexpr int add(int a, int b) {
+    return a + b;
+  }
+};
+struct B {
+  static int add(int a, int b) { // expected-note{{declared here}}
+    return a + b;
+  }
+};
+template<typename U>
+concept C4 = U::add(1, 2) == 3;
+// expected-error@-1{{substitution into constraint expression resulted in a non-constant expression}}
+// expected-note@-2{{non-constexpr function 'add' cannot be used in a constant expression}}
+static_assert(C4<A>);
+static_assert(!C4<B>); // expected-note {{while checking the satisfaction of concept 'C4<B>' requested here}}
+
+template<typename T, typename U>
+constexpr bool is_same_v = false;
+
+template<typename T>
+constexpr bool is_same_v<T, T> = true;
+
+template<typename T, typename U>
+concept Same = is_same_v<T, U>;
+
+static_assert(Same<int, int>);
+static_assert(Same<int, decltype(1)>);
+static_assert(!Same<int, unsigned int>);
+static_assert(!Same<A, B>);
+static_assert(Same<A, A>);
+
+static_assert(Same<bool, decltype(C1<int>)>);
+static_assert(Same<bool, decltype(C2<int>)>);
+static_assert(Same<bool, decltype(C3<int*>)>);
+static_assert(Same<bool, decltype(C4<A>)>);
+
+template<typename T> concept C5 = T{}; // expected-error {{atomic constraint must be of type 'bool' (found 'int')}}
+constexpr bool x = C5<int>; // expected-note {{while checking the satisfaction of concept 'C5<int>' requested here}}
+
+template<int x>
+concept IsEven = (x % 2) == 0;
+
+static_assert(IsEven<20>);
+static_assert(!IsEven<11>);
+
+template<template<typename T> typename P>
+concept IsTypePredicate = is_same_v<decltype(P<bool>::value), const bool>
+                          && is_same_v<decltype(P<int>::value), const bool>
+                          && is_same_v<decltype(P<long long>::value), const bool>;
+
+template<typename T> struct T1 {};
+template<typename T> struct T2 { static constexpr bool value = sizeof(T) == 2; };
+
+static_assert(IsTypePredicate<T2>);
+static_assert(!IsTypePredicate<T1>);
+
+namespace piecewise_substitution {
+  template <typename T>
+  concept True = true;
+
+  template <typename T>
+  concept A = True<T> || T::value;
+
+  template <typename T>
+  concept B = (True<T> || T::value);
+
+  template <typename T>
+  concept C = !True<T> && T::value || true;
+  // expected-warning@-1{{'&&' within '||'}}
+  // expected-note@-2{{place parentheses around the '&&' expression to silence this warning}}
+
+  template <typename T>
+  concept D = (!True<T> && T::value) || true;
+
+  template <typename T>
+  concept E = T::value || True<T>;
+
+  template <typename T>
+  concept F = (T::value || True<T>);
+
+  template <typename T>
+  concept G = T::value && !True<T> || true;
+  // expected-warning@-1{{'&&' within '||'}}
+  // expected-note@-2{{place parentheses around the '&&' expression to silence this warning}}
+
+  template <typename T>
+  concept H = (T::value && !True<T>) || true;
+
+  template <typename T>
+  concept I = T::value;
+
+  static_assert(A<int>);
+  static_assert(B<int>);
+  static_assert(C<int>);
+  static_assert(D<int>);
+  static_assert(E<int>);
+  static_assert(F<int>);
+  static_assert(G<int>);
+  static_assert(H<int>);
+  static_assert(!I<int>);
+}
+
+// Short ciruiting
+
+template<typename T> struct T3 { using type = typename T::type; };
+// expected-error@-1{{type 'char' cannot be used prior to '::' because it has no members}}
+// expected-error@-2{{type 'short' cannot be used prior to '::' because it has no members}}
+
+template<typename T>
+concept C6 = sizeof(T) == 1 && sizeof(typename T3<T>::type) == 1;
+// expected-note@-1{{while substituting template arguments into constraint expression here}}
+// expected-note@-2{{in instantiation of template class 'T3<char>' requested here}}
+
+template<typename T>
+concept C7 = sizeof(T) == 1 || sizeof(
+// expected-note@-1{{while substituting template arguments into constraint expression here}}
+    typename
+      T3<T>
+// expected-note@-1{{in instantiation of template class 'T3<short>' requested here}}
+        ::type) == 1;
+
+static_assert(!C6<short>);
+static_assert(!C6<char>); // expected-note{{while checking the satisfaction of concept 'C6<char>' requested here}}
+static_assert(C7<char>);
+static_assert(!C7<short>); // expected-note{{while checking the satisfaction of concept 'C7<short>' requested here}}
+
+// Make sure argument list is converted when instantiating a CSE.
+
+template<typename T, typename U = int>
+concept SameSize = sizeof(T) == sizeof(U);
+
+template<typename T>
+struct X { static constexpr bool a = SameSize<T>; };
+
+static_assert(X<unsigned>::a);

test/Parser/cxx2a-concept-declaration.cpp

 // Support parsing of concepts
 
 // RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -verify %s
 template<typename T> concept C1 = true; // expected-note 2{{previous}}
 
 template<typename T> concept C1 = true; // expected-error{{redefinition}}
 
 template<concept T> concept D1 = true;
 // expected-error@-1{{expected template parameter}}
 // expected-error@-2{{concept template parameter list must have at least one parameter; explicit specialization of concepts is not allowed}}
 
 template<template<typename> concept T> concept D2 = true;
 // expected-error@-1{{expected identifier}}
 // expected-error@-2{{template template parameter requires 'class' after the parameter list}}
 // expected-error@-3{{concept template parameter list must have at least one parameter; explicit specialization of concepts is not allowed}}
 
-template<typename T> concept C2 = 0.f; // expected-error {{constraint expression must be of type 'bool' but is of type 'float'}}
-
 struct S1 {
   template<typename T> concept C1 = true; // expected-error {{concept declarations may only appear in global or namespace scope}}
 };
 
 extern "C++" {
   template<typename T> concept C1 = true; // expected-error{{redefinition}}
 }
 
 template<typename A>
 template<typename B>
-concept C4 = true; // expected-error {{extraneous template parameter list in concept definition}}
+concept C2 = true; // expected-error {{extraneous template parameter list in concept definition}}
 
-template<typename T> concept C5 = true; // expected-note {{previous}} expected-note {{previous}}
-int C5; // expected-error {{redefinition}}
-struct C5 {}; // expected-error {{redefinition}}
-
-struct C6 {}; // expected-note{{previous definition is here}}
-template<typename T> concept C6 = true;
-// expected-error@-1{{redefinition of 'C6' as different kind of symbol}}
+template<typename T> concept C3 = true; // expected-note {{previous}} expected-note {{previous}}
+int C3; // expected-error {{redefinition}}
+struct C3 {}; // expected-error {{redefinition}}
+
+struct C4 {}; // expected-note{{previous definition is here}}
+template<typename T> concept C4 = true;
+// expected-error@-1{{redefinition of 'C4' as different kind of symbol}}
 
 // TODO: Add test to prevent explicit specialization, partial specialization
 // and explicit instantiation of concepts.
 
 template<typename T, T v>
 struct integral_constant { static constexpr T value = v; };
 
 namespace N {
-  template<typename T> concept C7 = true;
+  template<typename T> concept C5 = true;
 }
-using N::C7;
+using N::C5;
 
-template <bool word> concept C8 = integral_constant<bool, wor>::value;
+template <bool word> concept C6 = integral_constant<bool, wor>::value;
 // expected-error@-1{{use of undeclared identifier 'wor'; did you mean 'word'?}}
 // expected-note@-2{{'word' declared here}}
 
-template<typename T> concept bool C9 = true;
+template<typename T> concept bool C7 = true;
 // expected-warning@-1{{ISO C++2a does not permit the 'bool' keyword after 'concept'}}
 
-template<> concept C10 = false;
+template<> concept C8 = false;
 // expected-error@-1{{concept template parameter list must have at least one parameter; explicit specialization of concepts is not allowed}}
 
-template<> concept C9<int> = false;
+template<> concept C7<int> = false;
 // expected-error@-1{{name defined in concept definition must be an identifier}}
 
-template<typename T> concept N::C11 = false;
+template<typename T> concept N::C9 = false;
 // expected-error@-1{{name defined in concept definition must be an identifier}}
 
 class A { };
 // expected-note@-1{{'A' declared here}}
 
-template<typename T> concept A::C12 = false;
+template<typename T> concept A::C10 = false;
 // expected-error@-1{{expected namespace name}}
 
 template<typename T> concept operator int = false;
 // expected-error@-1{{name defined in concept definition must be an identifier}}
+
+template<bool x> concept C11 = 2; // expected-error {{atomic constraint must be of type 'bool' (found 'int')}}
+template<bool x> concept C12 = 2 && x; // expected-error {{atomic constraint must be of type 'bool' (found 'int')}}
+template<bool x> concept C13 = x || 2 || x; // expected-error {{atomic constraint must be of type 'bool' (found 'int')}}
+template<bool x> concept C14 = 8ull && x || x; // expected-error {{atomic constraint must be of type 'bool' (found 'unsigned long long')}}
+template<typename T> concept C15 = sizeof(T); // expected-error {{atomic constraint must be of type 'bool' (found 'unsigned long')}}
+template<typename T> concept C16 = T{};
+static_assert(!C16<bool>);
+template<typename T> concept C17 = (bool&&)true;
+static_assert(C17<int>);
+template<typename T> concept C18 = (const bool&)true;
+static_assert(C18<int>);
+template<typename T> concept C19 = (const bool)true;
+static_assert(C19<int>);
+template <bool c> concept C20 = integral_constant<bool, c>::value && true;
+static_assert(C20<true>);
+static_assert(!C20<false>);
+template <bool c> concept C21 = integral_constant<bool, c>::value;
+static_assert(C21<true>);
+static_assert(!C21<false>);
+
+template <bool word> concept C22 = integral_constant<bool, wor>::value;
+// expected-error@-1{{use of undeclared identifier 'wor'; did you mean 'word'?}}
+// expected-note@-2{{'word' declared here}}

tools/libclang/CXCursor.cpp

case Stmt::CoreturnStmtClass:
     break;
 
   case Stmt::ArrayTypeTraitExprClass:
   case Stmt::AsTypeExprClass:
   case Stmt::AtomicExprClass:
   case Stmt::BinaryConditionalOperatorClass:
   case Stmt::TypeTraitExprClass:
   case Stmt::CoawaitExprClass:
+  case Stmt::ConceptSpecializationExprClass:
   case Stmt::DependentCoawaitExprClass:
   case Stmt::CoyieldExprClass:
   case Stmt::CXXBindTemporaryExprClass:
   case Stmt::CXXDefaultArgExprClass:
   case Stmt::CXXDefaultInitExprClass:
   case Stmt::CXXFoldExprClass:
   case Stmt::CXXStdInitializerListExprClass:
   case Stmt::CXXScalarValueInitExprClass: