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

[Younan Zhang via cfe-commits]

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@@ -10827,8 +10865,16 @@ DeclResult Sema::ActOnExplicitInstantiation(Scope *S,
       return true;
     }
 
-    // FIXME: Create an ExplicitInstantiation node?
-    return (Decl*) nullptr;
+    const ASTTemplateArgumentListInfo *ArgsAsWritten = nullptr;
+    if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(Prev))
+      ArgsAsWritten = VTSD->getTemplateArgsAsWritten();
+    addExplicitInstantiationDecl(Context, CurContext, D.getCXXScopeSpec(), Prev,
+                                 ExternLoc, TemplateLoc, SourceLocation(),
+                                 ArgsAsWritten, D.getIdentifierLoc(), T, TSK);
+    // Don't return the EID to the Parser — doing so would trigger
+    // unrelated semantic actions (e.g. access checks via
+    // FinalizeDeclaration).
+    return (Decl *)nullptr;
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zyn0217 wrote:

Do you have a test if we return something?

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