[OpenMP][FIX] Verify compatible types for declare variant calls

Especially for templates we need to check at some point if the base
function matches the specialization we might call instead. Before this
lead to the replacement of `std::sqrt(int(2))` calls with one that
converts the argument to a `std::complex<int>`, clearly not the desired
behavior.

Reported as PR47655

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D88384

Author: jdoerfert

clang/lib/Sema/SemaOpenMP.cpp

@@ -5921,7 +5921,6 @@ void Sema::ActOnStartOfFunctionDefinitionInOpenMPDeclareVariantScope(
       continue;
 
     QualType UDeclTy = UDecl->getType();
-    // TODO: Verify types for templates eventually.
     if (!UDeclTy->isDependentType()) {
       QualType NewType = Context.mergeFunctionTypes(
           FType, UDeclTy, /* OfBlockPointer */ false,
@@ -6009,6 +6008,8 @@ ExprResult Sema::ActOnOpenMPCall(ExprResult Call, Scope *Scope,
   TargetOMPContext OMPCtx(Context, std::move(DiagUnknownTrait),
                           getCurFunctionDecl());
 
+  QualType CalleeFnType = CalleeFnDecl->getType();
+
   SmallVector<Expr *, 4> Exprs;
   SmallVector<VariantMatchInfo, 4> VMIs;
   while (CalleeFnDecl) {
@@ -6061,8 +6062,19 @@ ExprResult Sema::ActOnOpenMPCall(ExprResult Call, Scope *Scope,
       }
       NewCall = BuildCallExpr(Scope, BestExpr, LParenLoc, ArgExprs, RParenLoc,
                               ExecConfig);
-      if (NewCall.isUsable())
-        break;
+      if (NewCall.isUsable()) {
+        if (CallExpr *NCE = dyn_cast<CallExpr>(NewCall.get())) {
+          FunctionDecl *NewCalleeFnDecl = NCE->getDirectCallee();
+          QualType NewType = Context.mergeFunctionTypes(
+              CalleeFnType, NewCalleeFnDecl->getType(),
+              /* OfBlockPointer */ false,
+              /* Unqualified */ false, /* AllowCXX */ true);
+          if (!NewType.isNull())
+            break;
+          // Don't use the call if the function type was not compatible.
+          NewCall = nullptr;
+        }
+      }
     }
 
     VMIs.erase(VMIs.begin() + BestIdx);

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -444,6 +444,12 @@ static void instantiateOMPDeclareVariantAttr(
                                                          New->getLocation());
         if (!SubstFD)
           return;
+        QualType NewType = S.Context.mergeFunctionTypes(
+            SubstFD->getType(), FD->getType(),
+            /* OfBlockPointer */ false,
+            /* Unqualified */ false, /* AllowCXX */ true);
+        if (NewType.isNull())
+          return;
         S.InstantiateFunctionDefinition(
             New->getLocation(), SubstFD, /* Recursive */ true,
             /* DefinitionRequired */ false, /* AtEndOfTU */ false);