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

[Matheus Izvekov via cfe-commits]

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@@ -10827,8 +10869,19 @@ 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(),
+                                   D.getSourceRange().getEnd(), 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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mizvekov wrote:

You don't need to introduce this scope since we are at the end of the outer scope anyway.

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