Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3156,10 +3156,11 @@
   "__declspec attribute %0 is not supported">,
   InGroup<IgnoredAttributes>;
 def warn_attribute_has_no_effect : Warning<
-  "attribute %0 has no effect when annotating %select{an infinite loop}1">,
+  "attribute %0 has no effect when annotating %select{an infinite loop|"
+  "an 'if constexpr' statement}1">,
    InGroup<IgnoredAttributes>;
 def note_attribute_has_no_effect_here : Note<
-  "annotating the %select{infinite loop}0 here">;
+  "annotating the %select{infinite loop|'if constexpr' statement}0 here">;
 def err_decl_attribute_invalid_on_stmt : Error<
   "%0 attribute cannot be applied to a statement">;
 def err_stmt_attribute_invalid_on_decl : Error<
Index: clang/lib/Sema/SemaStmt.cpp
===================================================================
--- clang/lib/Sema/SemaStmt.cpp
+++ clang/lib/Sema/SemaStmt.cpp
@@ -597,16 +597,29 @@
     DiagnoseEmptyStmtBody(CondExpr->getEndLoc(), thenStmt,
                           diag::warn_empty_if_body);
 
-  std::tuple<bool, const Attr *, const Attr *> LHC =
-      Stmt::determineLikelihoodConflict(thenStmt, elseStmt);
-  if (std::get<0>(LHC)) {
-    const Attr *ThenAttr = std::get<1>(LHC);
-    const Attr *ElseAttr = std::get<2>(LHC);
-    Diags.Report(ThenAttr->getLocation(),
-                 diag::warn_attributes_likelihood_ifstmt_conflict)
-        << ThenAttr << ThenAttr->getRange();
-    Diags.Report(ElseAttr->getLocation(), diag::note_conflicting_attribute)
-        << ElseAttr << ElseAttr->getRange();
+  if (IsConstexpr) {
+    auto DiagnoseLikelihood = [&](const Stmt *S) {
+      if (const Attr *A = Stmt::getLikelihoodAttr(S)) {
+        Diags.Report(A->getLocation(), diag::warn_attribute_has_no_effect)
+            << A << 1 << A->getRange();
+        Diags.Report(IfLoc, diag::note_attribute_has_no_effect_here)
+            << 1 << SourceRange(IfLoc, LParenLoc.getLocWithOffset(-1));
+      }
+    };
+    DiagnoseLikelihood(thenStmt);
+    DiagnoseLikelihood(elseStmt);
+  } else {
+    std::tuple<bool, const Attr *, const Attr *> LHC =
+        Stmt::determineLikelihoodConflict(thenStmt, elseStmt);
+    if (std::get<0>(LHC)) {
+      const Attr *ThenAttr = std::get<1>(LHC);
+      const Attr *ElseAttr = std::get<2>(LHC);
+      Diags.Report(ThenAttr->getLocation(),
+                   diag::warn_attributes_likelihood_ifstmt_conflict)
+          << ThenAttr << ThenAttr->getRange();
+      Diags.Report(ElseAttr->getLocation(), diag::note_conflicting_attribute)
+          << ElseAttr << ElseAttr->getRange();
+    }
   }
 
   return BuildIfStmt(IfLoc, IsConstexpr, LParenLoc, InitStmt, Cond, RParenLoc,
Index: clang/test/SemaCXX/attr-likelihood.cpp
===================================================================
--- clang/test/SemaCXX/attr-likelihood.cpp
+++ clang/test/SemaCXX/attr-likelihood.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -fsyntax-only -verify
+// RUN: %clang_cc1 %s -std=c++17 -fsyntax-only -verify
 // RUN: %clang_cc1 %s -DPEDANTIC -pedantic -fsyntax-only -verify
 
 #if PEDANTIC
@@ -129,4 +129,23 @@
   catch (...) [[likely]] { // expected-error {{expected expression}}
   }
 }
+
+void o()
+{
+  // expected-warning@+2 {{attribute 'likely' has no effect when annotating an 'if constexpr' statement}}
+  // expected-note@+1 {{annotating the 'if constexpr' statement here}}
+  if constexpr (true) [[likely]];
+
+  // expected-note@+1 {{annotating the 'if constexpr' statement here}}
+  if constexpr (true) {
+  // expected-warning@+1 {{attribute 'unlikely' has no effect when annotating an 'if constexpr' statement}}
+  } else [[unlikely]];
+
+  // Annotating both branches with conflicting likelihoods generates no diagnostic regarding the conflict.
+  // expected-warning@+2 {{attribute 'likely' has no effect when annotating an 'if constexpr' statement}}
+  // expected-note@+1 2 {{annotating the 'if constexpr' statement here}}
+  if constexpr (true) [[likely]] {
+  // expected-warning@+1 {{attribute 'likely' has no effect when annotating an 'if constexpr' statement}}
+  } else [[likely]];
+}
 #endif