[flang][NFC] Document an intentional violation of the standard (#99073)

The Fortran standard committees passed an "interp" request at their June
2024 meetings that is distinct from nearly every other Fortran compiler
that I tried (6) in an an ambiguous case (parent component naming when
the base type has been renamed via USE association). Document this case
in flang/docs/Extensions.md as an intentional instance of
non-conformance chosen for portability and better usability.

Author: klausler

flang/docs/Extensions.md

@@ -134,6 +134,13 @@ end
   implicitly simply appearing in an asynchronous data transfer statement,
   without the attribute being visible in the procedure's explicit
   interface.
+* When the name of an extended derived type's base type is the
+  result of `USE` association with renaming, the name of the extended
+  derived type's parent component is the new name by which the base
+  is known in the scope of the extended derived type, not the original.
+  This interpretation has usability advantages and is what six other
+  Fortran compilers do, but is not conforming now that J3 approved an
+  "interp" in June 2024 to the contrary.
 
 ## Extensions, deletions, and legacy features supported by default
 

flang/test/Semantics/parent-comp-name.f90

@@ -0,0 +1,70 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+! Every other Fortran compiler (but one) interprets the names of parent
+! components like this when the names of their types are the product of
+! USE association with renaming.
+
+module m1
+  type originalName
+    integer m
+  end type
+end
+
+module m2
+  use m1, newName => originalName
+  type, extends(newName) :: extended
+    integer n
+  end type
+  type, extends(newName) :: extended2
+    integer originalName ! ok
+  end type
+ contains
+  subroutine s1
+    type(extended) x
+    type(extended2) x2
+    print *, x%newName%m ! ok
+    !ERROR: Component 'originalname' not found in derived type 'extended'
+    print *, x%originalName
+    print *, extended(newName=newName(m=1),n=2) ! ok
+    !ERROR: Structure constructor lacks a value for component 'm'
+    !ERROR: Keyword 'originalname=' does not name a component of derived type 'extended'
+    !ERROR: Keyword 'm=' may not appear in a reference to a procedure with an implicit interface
+    print *, extended(originalName=originalName(m=1),n=2)
+    !ERROR: Value in structure constructor of type 'REAL(4)' is incompatible with component 'newname' of type 'newname'
+    !ERROR: Keyword 'm=' may not appear in a reference to a procedure with an implicit interface
+    print *, extended(newName=originalName(m=1),n=2)
+    !ERROR: Structure constructor lacks a value for component 'm'
+    !ERROR: Keyword 'originalname=' does not name a component of derived type 'extended'
+    print *, extended(originalName=newName(m=1),n=2)
+    print *, x2%newName%m ! ok
+    print *, x2%originalName ! ok
+    print *, extended2(newName=newName(m=1),originalName=2) ! ok
+  end
+end
+
+module m3
+  use m2
+ contains
+  ! Same as above, but not in the same module as the derived
+  ! types' definitions.
+  subroutine s2
+    type(extended) x
+    type(extended2) x2
+    print *, x%newName%m ! ok
+    !ERROR: Component 'originalname' not found in derived type 'extended'
+    print *, x%originalName
+    print *, extended(newName=newName(m=1),n=2) ! ok
+    !ERROR: Structure constructor lacks a value for component 'm'
+    !ERROR: Keyword 'originalname=' does not name a component of derived type 'extended'
+    !ERROR: Keyword 'm=' may not appear in a reference to a procedure with an implicit interface
+    print *, extended(originalName=originalName(m=1),n=2)
+    !ERROR: Value in structure constructor of type 'REAL(4)' is incompatible with component 'newname' of type 'newname'
+    !ERROR: Keyword 'm=' may not appear in a reference to a procedure with an implicit interface
+    print *, extended(newName=originalName(m=1),n=2)
+    !ERROR: Structure constructor lacks a value for component 'm'
+    !ERROR: Keyword 'originalname=' does not name a component of derived type 'extended'
+    print *, extended(originalName=newName(m=1),n=2)
+    print *, x2%newName%m ! ok
+    print *, x2%originalName ! ok
+    print *, extended2(newName=newName(m=1),originalName=2) ! ok
+  end
+end