c++: overeager type completion in convert_to_void [PR111419]

Here convert_to_void always completes the type of an indirection or
id-expression, but according to [expr.context] an lvalue-to-rvalue
conversion is applied to a discarded-value expression only if "the
expression is a glvalue of volatile-qualified type".  This patch
restricts convert_to_void's type completion to match.

	PR c++/111419

gcc/cp/ChangeLog:

	* cvt.cc (convert_to_void) <case INDIRECT_REF>: Only call
	complete_type if the type is volatile.
	<case VAR_DECL>: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires36.C: New test.
	* g++.dg/expr/discarded1.C: New test.
	* g++.dg/expr/discarded1a.C: New test.

4 files changed, 54 insertions, 3 deletions

diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index c6b52f0..96abfae 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1252,7 +1252,9 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	tree type = TREE_TYPE (expr);
 	int is_reference = TYPE_REF_P (TREE_TYPE (TREE_OPERAND (expr, 0)));
 	int is_volatile = TYPE_VOLATILE (type);
-	int is_complete = COMPLETE_TYPE_P (complete_type (type));
+	if (is_volatile)
+	  complete_type (type);
+	int is_complete = COMPLETE_TYPE_P (type);
 
 	/* Can't load the value if we don't know the type.  */
 	if (is_volatile && !is_complete)
@@ -1412,9 +1414,10 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
       {
 	/* External variables might be incomplete.  */
 	tree type = TREE_TYPE (expr);
-	int is_complete = COMPLETE_TYPE_P (complete_type (type));
 
-	if (TYPE_VOLATILE (type) && !is_complete && (complain & tf_warning))
+	if (TYPE_VOLATILE (type)
+	    && !COMPLETE_TYPE_P (complete_type (type))
+	    && (complain & tf_warning))
 	  switch (implicit)
 	    {
 	      case ICV_CAST:
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires36.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires36.C
new file mode 100644
index 0000000..8d3a4fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires36.C
@@ -0,0 +1,16 @@
+// PR c++/111419
+// { dg-do compile { target c++20 } }
+
+template<class F>
+concept invocable = requires(F& f) { f(); };
+
+template<class F>
+concept deref_invocable = requires(F& f) { *f(); };
+
+struct Incomplete;
+
+template<class T>
+struct Holder { T t; };
+
+static_assert(invocable<Holder<Incomplete>& ()>);
+static_assert(deref_invocable<Holder<Incomplete>* ()>);
diff --git a/gcc/testsuite/g++.dg/expr/discarded1.C b/gcc/testsuite/g++.dg/expr/discarded1.C
new file mode 100644
index 0000000..c0c22e9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/expr/discarded1.C
@@ -0,0 +1,15 @@
+// PR c++/111419
+
+struct Incomplete;
+
+template<class T> struct Holder { T t; }; // { dg-bogus "incomplete" }
+
+extern Holder<Incomplete> a;
+extern Holder<Incomplete>& b;
+extern Holder<Incomplete>* c;
+
+int main() {
+  a;
+  b;
+  *c;
+}
diff --git a/gcc/testsuite/g++.dg/expr/discarded1a.C b/gcc/testsuite/g++.dg/expr/discarded1a.C
new file mode 100644
index 0000000..1c4dff4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/expr/discarded1a.C
@@ -0,0 +1,17 @@
+// A version of discarded1.C using volatile types.
+// PR c++/111419
+
+struct Incomplete;
+
+template<class T, int> struct Holder { T t; }; // { dg-error "incomplete" }
+
+extern volatile Holder<Incomplete, 0> a;
+extern volatile Holder<Incomplete, 1>& b;
+extern volatile Holder<Incomplete, 2>* c;
+
+int main() {
+  a; // { dg-message "required from here" }
+  b; // { dg-message "required from here" }
+  // { dg-warning "implicit dereference will not access object" "" { target *-*-* } .-1 }
+  *c; // { dg-message "required from here" }
+}