[Pattern Matching] Fix bug causing incorrect diagnostic

Incorrect diagnostic can be emitted when using statement
expressions as a pattern substatement in an inspect with
a trailing `void` return type.

Add a specific check for the void return type. All other
cases are handled when we check for convertibility later
in semantic analysis.

fixes llvm#19

Author: dansarginson

clang/lib/Sema/SemaExpr.cpp

@@ -13979,8 +13979,17 @@ ExprResult Sema::ActOnStmtExprResult(ExprResult ER) {
   // to perform implicit conversion during the copy initialization.
   if (getCurScope()->isPatternScope()) {
     InspectExpr *IE = getCurFunction()->InspectStack.back().getPointer();
-    if (IE->hasExplicitResultType())
-      T = IE->getType();
+    if (IE->hasExplicitResultType()) {
+      QualType ER = IE->getType();
+
+      // Catch this edge case where we can't make a valid
+      // statement expression result from a void type.
+      // All other cases should be caught elsewhere when
+      // we explore convertibility between the pattern
+      // substatement and the inspect return type.
+      if (!ER.getTypePtr()->isVoidType())
+        T = ER;
+    }
   }
 
   // FIXME: Provide a better location for the initialization.

clang/test/SemaCXX/inspect.cpp

@@ -17,7 +17,7 @@ void a2() {
 
 void b(int x) {
   inspect(x) -> void {
-    __ => 3; // expected-error {{cannot initialize statement expression result of type 'void' with an rvalue of type 'int'}}
+    __ => 3; // expected-error {{resulting expression type 'int' must match trailing result type 'void'}}
   };
 }