[flang] Accept TYPE(intrinsic type) in declarations only for non-extension type

[klausler]

Summary

To avoid clashing with names of user derived types, the redundant
syntax TYPE(intrinsic type spec) must be interpreted as a monomorphic
derived type when "intrinsic type spec" is a single word. This
affects TYPE(BYTE) and TYPE(DOUBLECOMPLEX), but not TYPE(DOUBLE COMPLEX)
in free form source.

[jeanPerier]

Looks good

flang/docs/Extensions.md

 
 ## Extensions, deletions, and legacy features supported by default
 
 * Tabs in source
 * `<>` as synonym for `.NE.` and `/=`
 * `$` and `@` as legal characters in names
 * Initialization in type declaration statements using `/values/`
 * Kind specification with `*`, e.g. `REAL*4`
-* `DOUBLE COMPLEX`
+* `DOUBLE COMPLEX` as a synonym for `COMPLEX(KIND(0.D0))` --
+  but not when spelled `TYPE(DOUBLECOMPLEX)`.
 * Signed complex literal constants
 * DEC `STRUCTURE`, `RECORD`, with '%FILL'; but `UNION`, and `MAP`
   are not yet supported throughout compilation, and elicit a
   "not yet implemented" message.
 * Structure field access with `.field`
-* `BYTE` as synonym for `INTEGER(KIND=1)`
+* `BYTE` as synonym for `INTEGER(KIND=1)`; but not when spelled `TYPE(BYTE)`.
 * Quad precision REAL literals with `Q`
 * `X` prefix/suffix as synonym for `Z` on hexadecimal literals
 * `B`, `O`, `Z`, and `X` accepted as suffixes as well as prefixes
 * Triplets allowed in array constructors
 * `%LOC`, `%VAL`, and `%REF`
 * Leading comma allowed before I/O item list
 * Empty parentheses allowed in `PROGRAM P()`
 * Missing parentheses allowed in `FUNCTION F`

flang/lib/Parser/Fortran-parsers.cpp

 // R703 declaration-type-spec ->
 //        intrinsic-type-spec | TYPE ( intrinsic-type-spec ) |
 //        TYPE ( derived-type-spec ) | CLASS ( derived-type-spec ) |
 //        CLASS ( * ) | TYPE ( * )
 // N.B. It is critical to distribute "parenthesized()" over the alternatives
 // for TYPE (...), rather than putting the alternatives within it, which
 // would fail on "TYPE(real_derived)" with a misrecognition of "real" as an
 // intrinsic-type-spec.
+// N.B. TYPE(x) is a derived type if x is a one-word extension intrinsic
+// type (BYTE or DOUBLECOMPLEX), not the extension intrinsic type.
 TYPE_CONTEXT_PARSER("declaration type spec"_en_US,
     construct<DeclarationTypeSpec>(intrinsicTypeSpec) ||
         "TYPE" >>
-            (parenthesized(construct<DeclarationTypeSpec>(intrinsicTypeSpec)) ||
+            (parenthesized(construct<DeclarationTypeSpec>(
+                 !"DOUBLECOMPLEX"_tok >> !"BYTE"_tok >> intrinsicTypeSpec)) ||
                 parenthesized(construct<DeclarationTypeSpec>(
                     construct<DeclarationTypeSpec::Type>(derivedTypeSpec))) ||
                 construct<DeclarationTypeSpec>(
                     "( * )" >> construct<DeclarationTypeSpec::TypeStar>())) ||
         "CLASS" >> parenthesized(construct<DeclarationTypeSpec>(
                                      construct<DeclarationTypeSpec::Class>(
                                          derivedTypeSpec)) ||
                        construct<DeclarationTypeSpec>("*" >>
first(construct<IntrinsicTypeSpec>(integerTypeSpec),
         construct<IntrinsicTypeSpec>(construct<IntrinsicTypeSpec::Complex>(
             "COMPLEX" >> maybe(kindSelector))),
         construct<IntrinsicTypeSpec>(construct<IntrinsicTypeSpec::Character>(
             "CHARACTER" >> maybe(Parser<CharSelector>{}))),
         construct<IntrinsicTypeSpec>(construct<IntrinsicTypeSpec::Logical>(
             "LOGICAL" >> maybe(kindSelector))),
         extension<LanguageFeature::DoubleComplex>(
             "nonstandard usage: DOUBLE COMPLEX"_port_en_US,
-            construct<IntrinsicTypeSpec>("DOUBLE COMPLEX" >>
+            construct<IntrinsicTypeSpec>("DOUBLE COMPLEX"_sptok >>
                 construct<IntrinsicTypeSpec::DoubleComplex>())),
         extension<LanguageFeature::Byte>("nonstandard usage: BYTE"_port_en_US,
             construct<IntrinsicTypeSpec>(construct<IntegerTypeSpec>(
                 "BYTE" >> construct<std::optional<KindSelector>>(pure(1)))))))
 
 // R705 integer-type-spec -> INTEGER [kind-selector]
 TYPE_PARSER(construct<IntegerTypeSpec>("INTEGER" >> maybe(kindSelector)))