[clang] [clang-tools-extra] [Clang][AST] Introduce `ExplicitInstantiationDecl` to preserve source info and fix diagnostic locations (PR #191658)

[Chuanqi Xu via cfe-commits]

================
@@ -9314,10 +9314,36 @@ static void StripImplicitInstantiation(NamedDecl *D, bool MinGW) {
     FD->setInlineSpecified(false);
 }
 
+/// Create an ExplicitInstantiationDecl to record source-location info for an
+/// explicit template instantiation statement, and add it to \p CurContext.
+static void addExplicitInstantiationDecl(
+    ASTContext &Context, DeclContext *CurContext, const CXXScopeSpec &SS,
+    NamedDecl *Spec, SourceLocation ExternLoc, SourceLocation TemplateLoc,
+    SourceLocation TagKWLoc, const ASTTemplateArgumentListInfo *ArgsAsWritten,
+    SourceLocation NameLoc, TypeSourceInfo *TypeAsWritten,
+    TemplateSpecializationKind TSK) {
+  NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  auto *EID = ExplicitInstantiationDecl::Create(
+      Context, CurContext, Spec, ExternLoc, TemplateLoc, TagKWLoc, QualifierLoc,
+      ArgsAsWritten, NameLoc, TypeAsWritten, TSK);
+  CurContext->addDecl(EID);
+}
+
 /// Compute the diagnostic location for an explicit instantiation
 //  declaration or definition.
 static SourceLocation DiagLocForExplicitInstantiation(
     NamedDecl* D, SourceLocation PointOfInstantiation) {
+  // Search for an ExplicitInstantiationDecl that references D. Per
+  // [temp.explicit], explicit instantiations must appear in an enclosing
+  // namespace of the template, so the EID is in D's DeclContext or an ancestor.
+  for (DeclContext *DC = D->getDeclContext(); DC; DC = DC->getParent()) {
+    for (auto *Decl : DC->decls()) {
+      if (auto *EID = dyn_cast<ExplicitInstantiationDecl>(Decl))
+        if (EID->getSpecialization() == D)
+          return EID->getTemplateLoc();
+    }
+  }
----------------
ChuanqiXu9 wrote:

I think we still need to reference it in the GMF. Otherwise  it will be optimized out and the intelligence won't get it. To confirm, you can try to see if it works correctly in intelligence as you expected.

https://github.com/llvm/llvm-project/pull/191658