Missing diagnostics "warning: type qualifiers ignored on function return type"

[pmor13]

typedef const int T;
T f(void);

$ clang -c -std=c11 -pedantic -Wall -Wextra
<nothing>

[shafik]

For C++ this will also miss this case:

typedef const int T;
auto f(void) -> T;

[shafik]

This is failing in diagnoseRedundantReturnTypeQualifiers(...) the step before in GetFullTypeForDeclarator(...) we are using getCVRQualifiers() to detect if there are cv-qualifiers, this looks through type sugar.

Inside diagnoseRedundantReturnTypeQualifiers(...) for the trailing return type case we use getLocalCVRQualifiers() which does not look through type sugar.

For the case reported by the OP we are using D.getDeclSpec().getTypeQualifiers() and the DeclSpec does look through the type sugar.

The problem is not simply looking through the type sugar, it is also getting a quality diagnostic. For example for the trailing return type case if we used RetTy.getCVRQualifiers() instead we would obtain a diagnostic like this:

warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
auto f2(void) -> T;
                 ^

which would not be too helpful.

[AaronBallman]

This issue is related to #39494