[clang] [Clang] Make the result type of sizeof/pointer subtraction/size_t literals be typedefs instead of built-in types (PR #143653)

[A. Jiang via cfe-commits]

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@@ -6726,17 +6726,67 @@ QualType ASTContext::getTagDeclType(const TagDecl *Decl) const {
   return getTypeDeclType(const_cast<TagDecl*>(Decl));
 }
 
+// Inject __size_t, __signed_size_t, and __ptrdiff_t to provide portable hints
+// and diagnostics. In C and C++, expressions of type size_t can be obtained via
+// the sizeof operator, expressions of type ptrdiff_t via pointer subtraction,
+// and expressions of type signed size_t via the z literal suffix (since C++23).
+// However, no core language mechanism directly produces an expression of type
+// unsigned ptrdiff_t. The unsigned ptrdiff_t type is solely required by format
+// specifiers for printf and scanf. Consequently, no expression's type needs to
+// be displayed as unsigned ptrdiff_t. Verification of whether a type is
+// unsigned ptrdiff_t is also unnecessary, as no corresponding typedefs exist.
+// Therefore, injecting a typedef for signed ptrdiff_t is not required.
+
 /// getSizeType - Return the unique type for "size_t" (C99 7.17), the result
 /// of the sizeof operator (C99 6.5.3.4p4). The value is target dependent and
 /// needs to agree with the definition in <stddef.h>.
-CanQualType ASTContext::getSizeType() const {
+QualType ASTContext::getSizeType() const {
+  if (SizeType.isNull()) {
+    if (auto const &LO = getLangOpts(); !LO.HLSL && (LO.C99 || LO.CPlusPlus))
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frederick-vs-ja wrote:

I wonder why C99 is required, given `size_t` and `ptrdiff_t` have been existing since C89.

https://github.com/llvm/llvm-project/pull/143653