From 611bd6e2dbf0acff3a80282675b7fcf3afe3ec84 Mon Sep 17 00:00:00 2001
From: Dan Sarginson <dan.sarginson@gmail.com>
Date: Thu, 4 Mar 2021 17:58:23 +0000
Subject: [PATCH] [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 #19
---
 clang/lib/Sema/SemaExpr.cpp    | 13 +++++++++++--
 clang/test/SemaCXX/inspect.cpp |  2 +-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 33354df69575c4..62e108ef07efb4 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -16744,8 +16744,17 @@ 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.
diff --git a/clang/test/SemaCXX/inspect.cpp b/clang/test/SemaCXX/inspect.cpp
index 8ed814d1556f84..c200405a7d6ae3 100644
--- a/clang/test/SemaCXX/inspect.cpp
+++ b/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'}}
   };
 }